The big problem with the existing audio files is the muffled sound. Be sure to have have the Dolby turned off on the tape player in order to make sure all the high frequencies are preserved. I'd just save the files as mono, down-sampled to 22.05KHz WAV or FLAC files. Then play around with the "Noise Removal" effect in Audacity Beta program (
http://wiki.audacityteam.org/index.php?title=Noise_Removal) to get a good sounding result. Keep copies of the unfiltered versions of the files in case you decide to make changes later. Try using the following settings:
Noise reduction (db): 9
Frequency Smoothing (Hz): 150
Attack/Decay time (secs): 0.10
Be sure to use the output RCA connectors (
http://en.wikipedia.org/wiki/RCA_connector) on the tape player as these are the "LINE OUT" connectors that provide a standard signal needed for the "LINE IN" connector on the digitizer (computer). Don't use the player's headphone jack as the voltage on this line is not standardized and is typically more distorted.
Whatever you get is bound to sound better than what is currently here. Having a little more hiss sound is fine if it results in a less muffled overall result.