With a computer of that vintage, I'd open 'er up and take a look around before doing much of anything else. My guess is that you will find that many electrolytic capacitors on the logic board have leaked, leaving conductive smudges on the board. Clean the board thoroughly, and replace the capacitors. Often, that fixes many "mysterious" behaviors, including weak or noisy (or no) sound.

As to the wobbly raster, that is something you ought to fix. A few minutes with a soldering iron (and fresh solder) will likely take care of it. Leaving it as is can lead to more serious damage (it's hard on the flyback transformer), so it's best to fix it now. After taking care of all of the hardware problems, then do a clean re-install of System 6 or 7. Choose the former for speed, choose the latter if the apps you want to run demand it.