Grounding the left sound channel on the Video Input Module card is one possible cause of this symptom. The Video Input Module card has an EMI shield around the S-video port that can touching the left sound channel (white) port. This sends the signal to the electrical ground, instead of the speaker.
To correct this, pull the card out of the computer, bend back the EMI shield prongs, so they are no longer touching the left sound channel port, then reinstall the card.
The following computers are in the Macintosh 630 family: Performa 630, Performa 635, Performa 636, Performa 637, Performa 638, LC 630, and Quadra 630.