Can only confirm what other's already found, this cap the C301 (1000uF) always seems to leak and continue to give low volume problem even after a full recap if this isn't cleaned good.
The picture shows the under side where this line goes all the way to the separate audio board,
and it's under UV light and revealing the electrolyte as a dark/purple tone, the board is luckily not
damaged the blackness goes away when cleaned (if you're lucky) unlike when looking on battery leakage on Amiga 500 backup cap on the external memory board....
Look in the middle beside the two resistors, four vias.