One additional tip, even if late in coming:
Always try to play back hi-fi VHS tracks using the same physical VCR that recorded them. If the original machine is no longer available, then try to find another unit of exactly the same make and model to play them with. You'll achieve the best results this way, both in terms of minimizing (or even outright avoiding) the subtle "motorboat" hum of VHS hi-fi's head switching noise, as well as in terms of general trackability. And if you can't find the original VCR that recorded your hi-fi tapes, or another unit of the same make and model, then at least try multiple random makes/models of other non-matching VCRs until you find a unit that tracks the best and exhibits the least motorboat hum.
The reason is this. VHS hi-fi tracks were an afterthought that never existed in the format's original design. To shoehorn hi-fi audio into the format in a way that wouldn't disrupt playback on older machines unaware of hi-fi audio, compromises and tricks had to be employed. Think of how color was effectively shoehorned into analog television video to create NTSC, but at the cost of requiring comb or notch filtering in receivers, and of poor color resolution compared to the luminance, etc. Hi-fi on VHS was made possible by literally "burying" the hi-fi tracks beneath the video tracks (i.e. two recordings done over the top of each other without an erase head wiping the first out prior to the second being laid down). The way they are made separately readable in spite of this is by using separate heads for the hi-fi audio with large azimuth differences versus those of the video heads; and also by employing FM modulation for the hi-fi tracks (to exploit its capture effect). Because of all this, hi-fi tracks were never ideal in terms of their robustness. Between the VCR used to record hi-fi tracks, and the one used to play them, there can't be any differences, even small ones, in the hi-fi head alignments/azimuths/drum placements, or you start hearing motorboat hum and/or experiencing tracking fragility. Another problem with hi-fi tracks on VHS is that the hi-fi heads were always sized to SLP track widths throughout the entire history of the VHS Hi-Fi format. So even if you had a 4 head VCR with wide video heads for SP mode and narrower video heads for SLP, the manufacturers never wanted to spend the money putting two sets of hi-fi heads (wide and narrow) on the helical drum for use when recording SP vs. in SLP. In short: record in SP mode, and you get hi-fi tracks as thin and as fragile as SLP tracks, period. This often means that, with time, as tightly wound tapes age and slightly stretch by microscopic amounts, it can become as challenging to track the hi-fi audio on VHS as SLP video, even when playing an SP mode recording.
So, assuming you still have any of the tapes you were transferring before because you weren't 100% satisfied with the results, consider a second attempt with the original VCR(s) if you have them, or even by finding the same makes/models (if you remember them) on eBay and trying with those. You may get better results -- assuming you didn't get lucky and pry satisfactory transfers off of your tapes already with whatever you had at hand.
If you never saw
this thread, it features an active example of the phenomenon I'm describing with inter-machine incompatibility. (And alas, I still haven't gotten around to fixing up that old PV-S4670.)
