I just received an e-mail back from that Charles guy. Quick response I must say! 
Anyhow, all four of them have audio issues. Crackling and all that jazz. The PAL 600 needs it's PCMCIA port replaced. The NTSC 600 needs it's IDE port replaced. The PAL 1200 just needs recapped as far as I can tell. And the NTSC 1200 needs it's kickstart sockets replaced...
...a result of me trouble shooting a purchase from Amigakit.
The audio problems you're experiencing could be caused by bad capacitors and/or electromagnetic interference. Check for any bulging caps and for any brown rust-like material on, under, or near the capacitors. Not all capacitors that have gone bad will show physical signs, but it doesn't hurt to check since it is a sure sign that they have gone bad. The IDE port is easy to fix and the PCMCIA isn't that hard but takes more time.
Also, bad traces can also cause problems, especially on older motherboards. If you have a USB microscope and a multimeter, look for any corrosion and check the continuity between traces. Even if there is a loss of connection between traces due to corrosion it is still possible to fix it.