I'm pretty sure the 3rd party ROM SIMM is one of the ones I built, since I just received a similar question from Matt in my email today 
Replying here too, just to add my two cents to the discussion. I very much doubt that the ROM SIMM itself caused any damage to the sound circuit. It's not necessarily a coincidence though--maybe just the act of moving the logic board around to install the new ROM SIMM did something.
You mentioned that sound works great through the headphone jack still. I think that's a huge clue. It probably means the sound chip is fine. I actually had a very similar problem with my IIci (the first machine I ever recapped) earlier this year -- randomly one day I noticed the sound was acting up. It got worse and worse, and eventually stopped working at all. I decided to check the headphone jack one day, and surprisingly it worked 100% fine.
It turns out that the problem was capacitor goo which had leaked underneath the headphone jack. I had carefully removed all the capacitors and cleaned beneath them, but I didn't remove any nearby chips/components to clean underneath them. At least on the IIci, the speaker circuit goes through the headphone jack. When the headphones are removed, it sends the sound to the speaker. When the headphones are plugged in, it sends the sound to the headphones. On my IIci, I removed the headphone jack and immediately realized that the goo had eaten through one of the headphone jack's pins. It fell off as I removed the jack. It was absolutely filthy under all of the ports. Several traces on the serial ports were broken too. I made sure all traces were good/fixed, then I soldered a capacitor leg onto the remains of the broken pin on the audio jack (which was VERY difficult, BTW) and soldered the jack back onto the logic board. Now the sound coming from my speaker is perfect again. The broken pin was one of the pins involved in the circuit on the jack that hooks up the speaker when the headphones are unplugged. Over the past 2-3 years, the goo had slowly eaten away at that pin.
There are certainly a lot of possibilities for what might be wrong. My (potentially crazy) theory is that when removing the logic board to install the custom SIMM, a pin that was almost eaten through finally gave way and became an intermittent connection, and quickly became a fully broken connection after that.
Replying here too, just to add my two cents to the discussion. I very much doubt that the ROM SIMM itself caused any damage to the sound circuit. It's not necessarily a coincidence though--maybe just the act of moving the logic board around to install the new ROM SIMM did something.You mentioned that sound works great through the headphone jack still. I think that's a huge clue. It probably means the sound chip is fine. I actually had a very similar problem with my IIci (the first machine I ever recapped) earlier this year -- randomly one day I noticed the sound was acting up. It got worse and worse, and eventually stopped working at all. I decided to check the headphone jack one day, and surprisingly it worked 100% fine.
It turns out that the problem was capacitor goo which had leaked underneath the headphone jack. I had carefully removed all the capacitors and cleaned beneath them, but I didn't remove any nearby chips/components to clean underneath them. At least on the IIci, the speaker circuit goes through the headphone jack. When the headphones are removed, it sends the sound to the speaker. When the headphones are plugged in, it sends the sound to the headphones. On my IIci, I removed the headphone jack and immediately realized that the goo had eaten through one of the headphone jack's pins. It fell off as I removed the jack. It was absolutely filthy under all of the ports. Several traces on the serial ports were broken too. I made sure all traces were good/fixed, then I soldered a capacitor leg onto the remains of the broken pin on the audio jack (which was VERY difficult, BTW) and soldered the jack back onto the logic board. Now the sound coming from my speaker is perfect again. The broken pin was one of the pins involved in the circuit on the jack that hooks up the speaker when the headphones are unplugged. Over the past 2-3 years, the goo had slowly eaten away at that pin.
There are certainly a lot of possibilities for what might be wrong. My (potentially crazy) theory is that when removing the logic board to install the custom SIMM, a pin that was almost eaten through finally gave way and became an intermittent connection, and quickly became a fully broken connection after that.