The recorded sound playback on my vcr is hardly audible?

Make: Sanyo VCR (Unsure of specifics. Model approximatley 8 year old.).

The recorded sound playback on my VCR has been failing for the past month. (The picture quality is excellent).

Sometimes you can hear the sound with the volume turned right up, alternatively sometimes it is completely inaudible.

I have cleaned the heads with a good head tape cleaner, without effect. Equally, I have opened the machine and manually cleaned the heads, which had limited effect.

Can you please advise why the recorded sound playback is so poor? I cannot think of anything else to remedy the problem.