I've tested it on my own phone (Lumia 920), and a background audio recording uses about a half to a quarter as much power as the Gmail (IMAP) app - it's pretty negligible. Compressing it to send over the wire might use more (I was recording to WAV), but with the right choice of codec (e.g. GSM-HR as mentioned below) I can't see it adding all that much. A common complaint is that the Facebook app uses a lot of battery anyway, so the costs of audio recording probably _could_ (I doubt they _are_) be buried in that.