As a museum professional who actually provides small run customised audio tours (amongst other things - see my website www.hrec.ndirect.co.uk ) I would add the following observations.  Audio tours increasingly are on CD, minidisc and solid state (wands etc).  The random track access of this type of media is a definite plus for museums.  Guides allow a set route to be created and visitor numbers managed. Yes, using headphones does isolate other members of a group.  Increasingly in the UK low output portable 'ghetto blaster' CD systems are being used for replay. They are cheap and do not commit museums to expensive contracts with companies. (There are some disadvantages though) Most of my audio guides are for the visually impaired and other disadvantaged groups.  The approach is different and the scripting different to commercial products. Small museums might only need 1 or 2 CD's.
 
Sound replay systems for out of doors use will tend to more expensive than an audio tour operation, although it does depend on visitor numbers to some extent. Outdoor units need power, have to be what I call 'ice-cream proof', reliable and weatherproof. I have installed many such systems in castle ruins, preserved railways etc., some of them utilising stereo to great effect.
 
It is too big a subject to discuss in detail here.  If there are specific queries, I'll try and answer them on the Museums Discussion List or by e-mail [log in to unmask] 
 
Ken Howarth Heritage Recording (UK) Conwy, North Wales, UK.