Are you referring to Dolby B noise reduction found on certain VCRs?
To counteract tape hiss, decks applied Dolby B noise reduction for recording and playback. Dolby B dynamically boosts the mid-frequency band of the audio program on the recorded medium, improving its signal strength relative to the tape's background noise floor, then attenuates the mid-band during playback. Dolby B is not a transparent process, and Dolby-encoded program material will exhibit an unnatural mid-range emphasis when played on non-Dolby capable VCRs.
To my knowledge, you must find another Dolby deck. There's no other way to avoid the audio not sounding like crap.
I'm also under the understanding that this applies to mono, not just the HiFi/stereo tracks. (It's been years, however, since I had anything to do with Dolby NR, so I may not be remembering correctly on this mono/stereo issue.)