On the soundblaster Pro 1, one OPL2 chip generates sound for the left channel and the other one generates sound for the right channel. In mono playback, the SB Pro hardware mirrors every write to the OPL chips so both get programmed the same way. I recommend you to carefully listen whether in mono playback, both channels are sounding fine, or you just don't notice the issue of one channel being broken, because the missing or shortened note are still played on the other channel.

If mono playback is really fine, the OPL2 chips are likely fine. as the damage only covers serial ports and the sound card, it is likely that the issue was on the +12V or -12V line, most likely -12V. The -12V supply does not connect to the OPL2 chips, but either that line or the -5V line (which is generated from -12V in many supplies) is used in the analog circuits processung the OPL2 output signals. Given your description, I think a broken operational amplifier that handles FM music is more likely than a broken synthesizer chip.