If its a UK model, ensure the scart lead is pushed in firmly at both the VCR and TV end, they tend to work loose..a very common problem.
For all machines..another common cause is damage to the audio track. To check:
Press the button on the side of the cassette to release the flap covering the tape and carefully inspect the tape edge for damage. It must be perfect and straight. If it appears curled or a fine line runs along the tape edge, then the mechanism may be damaging your tapes, due to wear, bent or damaged tape guides or oxide deposits.
Cobweb, May 2005