The caps look visually okay and the 6100 is less prone to cap leakage than most, but that doesn't mean they're still not bad. Since your audio does still work, the fact it's low even at max, I can almost certainly say it's cap related. I can't think what else could cause that issue, other than damaged audio traces (let's hope it isn't that).
Even if it was fine, all the beige machines that exist that haven't been recapped in the past 15+ years will be certainly due for one.