Tried another recording of the tape, this time at 22hz though for me that made no difference sadly.
are these standard audio tapes?
remember volume can be important (you typically want as high as possible *without* clipping)
Sample at whatever the highest setting your input device has, if you're using something with fancy equalizer settings, make sure they're all turned off etc. as bass enhancements and fake spatial effects aren't going to do us any good here as they're more likely to corrupt the data ;-)
try a different cassette deck etc.
most cassette media, unless it was well stored, is gong to be in a bad state at this point unfortunately.
sometimes with these old games, the game was recorded on the b-side as well, so if one copy becomes corrupt the other might not be (no idea if Japan did this too)