know four or five distinct technologies for audio tours:

1) Portable Cassette tape players.  Pro: cheap marginal cost for production
and changes; con: linear tour path.

2) Portable CD Players.  Pro: non-linear; relatively inexpensive player.
Con: very expensive to revise tours; player is mechanical liable to
breakdown and theft.

3) Portable Digital EEPROM storage.  Pro: non linear, cheap to revise,
customized tours, no reason to steal the players.  Con: Expensive technology
to rent/lease/sole source (the Acoustiguide Wand I believe is the only one
of its kind)

4) Exhibit stations with broadcast audio.  This has a few subsets: digital
vs. analog source; infrared vs. radio spectrum broadcast.  The coolest
implementation of this I've seen used digital and multiplexed radio
broadcast, where up to seven people could arrive at the station at different
times and the audio program would begin at the beginning for each.
Extremely customizable, with inexpensive reprogramming and inexpensive
"players" this is good, if underused technology.

At the Hall we are using the Wand from Acoustiguide for our NSF-funded
evaluation of audio tours for hands on science centers (with the Lawrence
Hall of Science.)  We are also planning to use them for a new project, also
funded by NSF, to create audio tours for visually impaired visitors to
science centers.

Let me know if you want more info.

Eric