If it's the stereo track then I would expect bad picture too so I assume that its the mono sound. VCRS have a separate mono sound head so if it is mono then that head is broken, out of line, or dirty. Try a VCR cleaner. If that doesn't fix it try a new post here.
It's not worth having VCRs repaired professionally. I'd keep it for viewing old tapes and get a PVR or DVD recorder for off-air recording.
Terry
September 2006