This is one of those great places that you think you've just discovered and then find every other visitor knows it. You get live music in a cool cavernous setting. Plenty of locals seem to go there too. Easily missed - just a small door in the wall and not much to advertise it. Especially good to hear a bit of flamenco, with some authentically gurning singers. Free entry.
Calle Levies - in the Barrio S Cruz. Best to ask someone
Updates
These days, at times, La Carboneria can be packed full of teenage tourists. The best music is often earlier (around 11pm) in the first, smaller room before the place really fills up. Yes, it is pretty touristy : most folk seemed to have camcoders! But despite that, excellent atmosphere, and nice cool tree-filled courtyard for relaxed drinks. (Music is indoors and started around 11:30). Just round the corner is Cafe Levies: excellent outdoor cafe in a square with, unusually, several veggie options.
Is this your business?