Cassettes
Of course, Sam, if you're like me, you have a cassette player in every car and several in the house and barely have the option of playing a CD, let alone MP3 stuff.
High speed cassette duplicators made by companies such as Telex and 3M can dupe a C-60 cassette in 2 or 3 minutes, recording the duplicates and running the master at speeds approaching the rewind speed of el cheapo cassette players, doing both sides at the same time. The resulant copies may not be the best, but they aren't bad. The duplicators are used all the time by churches and training organizations.
Doing it with a regular cassette deck? Not easy. There are some dual decks out there that will dub at 2x speed with a slight loss in the highs. Your problem is that the frequency response of the heads has to be astronomical when you do higher speeds or you lose the highs. For instance, if a recorder normally can handle 11KHz with no problems, if you try using it at 5X for faster duplicating, it'll have to handle 55KHz everywhere -- in the playback heads, record heads and all the electronics. Most recorders' amplifiers have limited frequency response and their heads are much worse, so there'll be problems. If the cassettes are voice only, you have a better chance of making useable dupes, where the highs don't go much over 3KHz.
Dean