Doctor badly needed !

I have some 4 to 5 hours of conversation taped in 1977 with a cheap Philips cassette mono tape recorder. Neither the microphone not the recording quality were great, but the recorded material is of the outmost importance to me. Further, at one moment the batteries of the recorder went low, adding distortion to part of the registration. The only means I have today for transferring these 30 year old cassettes to my PC is a Sony Walkman through the earphone line out.

This material needs to be doctored to become audible, but I am quite an ignorant as to how to use Audacity and what filter to apply. At present … in my poor words… the voice lacks presence, seems as if chocked, going from too low to too high and as if it had been recorded in a empty room with echo… as for the part when the batteries got low, it just needs to slow down the tape to recover its “normality”.

I am attaching a samples of these tapes (all french language, sometime with two voices) and hope somebody might be able to help me in telling me what to do.