Hi! This is my first post to this group, so please bear with me. :)
I collect different old computers, and this has resulted in me
getting three different C64 units and a 128D. About a year ago, the
C64s were working beautifully, but when I booted them up yesterday
two of them had sound problems, much to my dismay.
Hopefully this isn't a failing SID, but I don't know how to diagnose
or repair (e.g. replace -.-) in that event.

What happens (C64):
The C64 seems to feel everything's normal, but only one or two of
the three audio channels plays. Sometimes one cuts in for a moment,
then blips and dies again. Both machines exhibit this, but on
different channels. The remaining channel plays perfectly with no
error. One of the two plays the additional channels at next to no
volume, but the result is it might as well be gone. The third system
shows no such glitch despite having been stored with the other units.

As for the 128D, it's never quite been working. It runs beautifully
in C64 mode without a hitch, but crashes when it starts up in 128.
More specifically, it seems to die 80% of the time you attempt to
run a BASIC statement or anything more than just leaving the machine
to sit at the BASIC start screen. I tend to guess it's a memory chip
that isn't mapped in on C64 mode, but I don't know how to go about
finding out.

Any suggestions on probable causes, troubleshooting, and subsequent
repair? I'm willing to consider paying for someone to repair the
systems, but the only place semi-locally that did repair on these
machines closed a couple of years ago. (An enthusiast doing it as a
sideline)
Thank you!

Supra