I've been experimenting today with http://www.evoca.com/
A free account allows you to record and store up to 60 minutes of sound. This interface is simple to use. Sound can be captured from a number of sources but the easiest is directly from a microphone on the computer. Everything is done through the web interface. When the recording has been made, an HTML snippet is provided to paste where you need it, and this invokes a flash player.
I'm trying these as an addition to my blog for the DSA module
I suppose it will just pollute the airwaves, but maybe it offers another mode for information dissemination which may just help some students. For two lectures last year I recorded the whole lecture and mounted it as an MP3 file, but they very long and un-synchronised with the Powerpoint slides. I tried calling out slide numbers but it was reminiscent of the early soccer commentaries which used numbered squares (the origin I learnt today of the phrase 'back to square 1') .