There's a website which pays you to review unsigned music. SliceThePie pays you 5p for each song you rate, but as you build up a reputation you can command up to 25p a time. You'll need a pretty open mind, as you can't choose specific genres, but once you've found a band you like you can vote for them and even buy stock in them to trade.
The system could also be profitable if you're a musician, as if your songs get voted for you could be financed for an album. The system's too complex to describe fully here, so read the site's tutorials before you start.
How much? A dedicated fan putting in a couple of hours a night could expect around £30/month. Not much, but not bad for a hobby. For a band, the sky's the limit if you're good enough.