As John would say "it's hosed"

Basically you have a (extremely common) board fault.
Could be RAM or ROMs or logic or a bad connection/bad socket(s) or broken track(s).
If you have no idea it may be better to look for someone who can do repairs and send it to them.
If you want to fix it yourself you need to buy some basic equipment and research info on doing PCB repairs for at least 1 month or 1 year or 10 years. Watch PCB repair vids on youtube (Adam has a few good ones), read books about PCB repairs, including arcade manuals and figure out how to read schematics. Read books/datasheets about how logic chips work. etc etc etc.
Then get a junker PCB and practice on it.
At the very least you need a logic probe, multi-meter, pointed cutters, soldering iron, solder sucker, new DIP sockets, EPROM programmer and preferably schematics for the board you want to work on.
As a starter check the ROMs against the known good MAME ROM-set using EPROM programmer.
If good, check clocks and activity using logic probe.
If good, check RAM.
If good you have a logic problem or broken track.

This post really needs to be stickied. The basic repair info above is common to almost every "My board doesn't work, how can I fix it?" type of question that gets asked all over the net.

that's what MAME is for. MAME has basically EVERY arcade game ever made.
all the roms you need are in the rom set for Defender.
google 'mame rom defender'
the first hit is the site you want.
navigate the site and download the roms from there.
if you can't figure it out tell me the location of the ROM and I'll make it available.
I have no idea what a "video sound rom" is.
either you want video roms or sound roms. the words 'video' and 'sound' are mutually exclusive.

Dirt Dash is Namco Super System 22. 4 boards plugged into in a metal cage with custom connectors. All the PCBs are surface mounted and there's tons of RAM on each PCB. It's impossible to work on it while powered up because the boards are not accessible and it won't work at all if even one PCB is removed.
Super System 22 is a pig to repair. I know, I have all of the SS22 games and a very large box full of spare PCBs).
Your only course is find another working board set or buy the individual PCBs and swap them one at a time.
All the PCBs are identical across all of the games on that system so you can use any board from any game, but the ROM board is all surface mounted so you can't just swap ROMs, you must swap the entire PCB.
You can swap the small program ROM board that plugs into the CPU board and the EPROMs and protection chip which are all socketed to another CPU board and try that. If still no luck, swap out each of the other PCBs (video board, DSP board).
Those PCBs are really only repairable by the most die-hard repair experts... and no that's not including me ;-)

you've just got a classic logic fault. it's very common to see old 80's PCBs failing with logic faults or RAM faults.
watch Adam's Repair 101 video's on CPU/logic theory and start checking logic with logic probe.
you could also start at the CPU and check you have a clock on pin 6 and reset signal (high to low transition on bootup) on the reset pin.
otherwise, you have a common logic fault....

check the color PROM (82S123 @6L) and the nearby logic chips.

Space Invaders uses multiple LM3900's (from memory about 6 or 7).
Try replacing all of them. That will likely fix your saucer sound too.
Otherwise you have a logic fault. Not surprising really for a game that's nearly 40 years old ;-)
There's not that many logic chips, you could just change all of them in shotgun style and as a side effect it'll bullet-proof your board for the future too.
Be sure to replace the logic chips with new parts and test them in an EPROM programmer to make sure they really are good.
Also use machine pin sockets, not cheap dual-wipe sockets.... Space Invaders deserves the best ;-)

well think about it for a minute, your PCB is almost 40 years old....
probably you have a logic fault with the RAM address/data logic.
I think the schems are available so study the schems to find the chips connected to the RAM and start there.
work backwards checking input/output pins on the logic chips until you find the issue.

look for a DIP chip with a shiny white label on it.
under the label it will be a PIC microcontroller, likely a PIC16C57 or something similar.

Its a really simple PCB with just ROM/RAM/CPU and logic so it's very easy to fix and a perfectly good example of something to learn on..... and as long as the PCB is in good condition it's probably worth spending time on it.
The watchdog means the CPU is trying to run the program but there's a problem, either the code is corrupt or it's waiting for something to happen and it takes too long. So the watchdog is working and you don't need to mess with it.
Check/re-seat ROMs.
Check for a clock at the crystal and on the Z80 at pin 6.
Replace the work RAM (the RAM near the Z80)
Check/replace Z80 and/or check/find logic fault. Start at the CPU and work backwards to get to the address logic/ROM/RAM etc inter-connections.
The CPU circuit on Pacman is not very big so if you follow the theory behind the Repair 101 videos by Adam you should be able to figure it out in time.

check here....
http://www.johnsarcade.com/forum/index.php?topic=3180.0

Go to TEST mode while the game is in attract mode.
Go into 'GAME OPTIONS' and change 'SOUND OUTPUT' from STEREO to MONAURAL.
Likely it is already set to MONO and you don't need to do anything.

for sure you've lost one of the colors.
go to the test mode and select the color test.
if you have a missing color you will see only black where the color bar should be for that color.
anyway, the board is JAMMA. Try another JAMMA PCB. If you still have bad colors it's the monitor.

On Frogger the sound is generated on the top board.
Assuming the AMP is getting 12v, wet your finger and touch the underside of the PCB where the amp is multiple times.
Touch all of the pins and somewhere you will hear a pop sound.
If no pop the AMP IC may be bad or the electrolytic caps that connect the AMP IC output pin to the edge connector pin (which goes to the speaker) may be bad.
If you hear a pop, re-seat the ROMs and/or replace the sockets.
If that doesn't help you have a RAM/CPU or logic fault.
The PCB is pretty easy to fix.
Do this in order (on the top board)....
Clean 18-pin edge connector both sides
Re-seat ROMs
Replace sockets
Replace RAM
Replace Z80
Find logic fault

It's probably a logic fault or video ram fault. It's fairly easy to fix those old TTL games.
First re-seat all the ROMs. Then swap out the video RAM (the 8216 RAMs just below the ROMs on the lower PCB) and/or the 2114 RAMs where-ever they are.
After that you have a logic fault.
It's old early hardware and was bootlegged to the max so the schems may be available.

You have sound so it's not voltage/wiring or PCB.
It's either a dip switch setting or the ROMs you have are a different revision.
Check all the versions of that game in MAME and burn new program ROMs for the version you want and that may fix it.
You just need to burn new program ROMs not everything.
Also, look on the PCB (or in the manual/PDF). The PCB may have another connector for 2nd speaker to give stereo and the missing sound may be coming out of that (currently unconnected) speaker.