In order to answer that question, it is necessary to explain a little about how VCRs handle audio. I'll try to keep this as simple as possible.
Most VCRs today actually record audio 2 different ways, usually called "Normal" and "HiFi" audio. Both are automatically recorded on consumer VCRs, but on professional models like duplicator and editing VCRs, they must be specifically set.
The names "Normal" and "HiFi" tell us very little about what is actually going on with the audio signal. The Normal audio is recorded on the video tape in a linear fashion, much like an audio cassette tape. As the tape passes a stationary record head, electromagnetic soundwaves are transferred to the tape. The Normal audio may be either mono or stereo, depending on your machine, though most consumer VCRs make this a mono signal. Due to the nature of this recording process, the Normal audio, when played back, has a limited frequency response, limited dynamic range, and more tape hiss than does the HiFi audio. (Frequency response is the range of sounds from deepest bass to highest sparkling treble. Dynamic range is the range of volume, from the lowest whisper to the loudest crash.)
The HiFi audio is a very different animal. Without getting too technical, these stereo audio tracks are put onto the tape in much the same way as is the video signal. Both the video and the HiFi audio heads are mounted on a small, fast-spinning drum inside the VCR which rotates at a slight angle to the tape path. This causes information to be recorded onto the tape in diagonal stripes, with each stripe representing a tiny fraction-of-a-second's-worth of video and audio. As you can see, this is a much more complicated method of recording, but it results in superior quality sound reproduction - when everything is working exactly right - which it does most of the time.
OK. So now you have an idea of how audio is recorded on a VCR. You know that there may be two different kinds of audio existing simultaneously on your tape. So which do you want and how does the VCR know which one to play for you? VCRs can sense whether HiFi tracks are present on the tape and will automatically play those tracks, because they usually have the best sounding audio, and that's usually what we want. If the VCR can't find HiFi tracks, it switches to the Normal audio track.
Sometimes, though, things can go wrong this system, either during the recording process or during playback, that mess up the audio that the VCR is trying to play.
The most common problem is poor tracking. When a tape is not able to track well, the VCR can "lose" the HiFi tracks. As a result, the VCR may switch back and forth between Normal and HiFi audio, or it may play just the Normal audio, or just the HiFi audio but with distortion or crackle, or worst case, it may not play any audio at all.
This may be resolved by finely adjusting the tracking control if your VCR has one (sometimes this feature is only available from the remote, and sometimes it is fully automatic and not user adjustable). Another method is to force your VCR to play the Normal audio. There may be a physical switch on the VCR for that, or, more commonly, it must be set from the on-screen menu.
EMPTY HIFI TRACKS
Sometimes a problem occurs during the recording of a videotape. On professional machines it is possible, either by a technical glitch or human error, to record "silence" on the HiFi tracks. In other words, the HiFi tracks were set to record, but no audio signal was being sent to them. So, your VCR senses that HiFi tracks are present, but there is only "dead air" on them. For this problem you must force your VCR to play the Normal audio.
ONLY ONE TRACK RECORDED
There are two HiFi tracks, a left/right stereo pair. If for some reason only one of those tracks has an audio signal recorded onto it, you may not hear any audio at all. I have seen this happen when a VCR is not properly wired to the video monitor or playback system. If only one of the stereo audio jacks is wired into the playback system, the other channel may not be heard.
There are three ways to correct this. The best way is to properly wire the outputs of the VCR: left-out of the VCR to left-in on the monitor, and right-out of the VCR to right-in on the monitor. It sounds simple, yet is often done wrong. Another solution is to force the Normal (mono) track to playback as described above. The third fix is to manually set the VCR to play only the left or only the right track through both left and right output jacks. There may be a switch or button on a some sophisticated VCRs for this, or it may be found in the on-screen menu under "audio" or "audio monitor."
Finally, with all electronic devices, there is always the possibility that it could develop some electronic intermittency. If the same problem happens with various tapes, a bad connection or internal electronic glitch could be the culprit. First check your wires, gently wiggling them to see if that makes any difference, and if so, replace them. (In the meantime, you can temporarily tape them down in a position that works.) If it is not the wires, try tapping on the VCR itself. If that makes a difference, then it's time for a new VCR.
That's it. You should now be able to correct 99% of your VCR audio playback problems.