I own a VIC-20 and use a video monitor instead of a TV. However, the monitor is video only, so I can't hear the sounds in my programs. My monitor cable has an audio output plug, but no one—not even Commodore—has been able to give me exact instructions on how to interface for audio. I have been told I need a high-impedance audio amplifier, but have been given no definition of what that means.
If you have a home stereo system, you probably have the high-impedance audio amplifier you need. Simply plug the audio output jack on your monitor cable into the auxiliary input jack of your stereo (use an extension cable if necessary). You might also want to use a Y-adapter, which feeds the single input from the computer into both of your stereo inputs. This should provide superb sound quality. As an alternative, many electronics stores such as Radio Shack sell small battery-powered amplifiers with a built-in speaker. The input jack on the small amplifier may not accept the plug on your monitor cable, so an adapter may be required.