I bought one of these tiny audio varios from an ultralight store online. It is designed for paraglider and hang glider pilots, but I found it works pretty well for sailplanes too! It is very small and light, and has sixteen different levels of sensitivity. Great for a backup or a club glider with no audio. Obviously it's not TE compensated, but I found that it was chirping at me pretty much at the same time as the Cambridge vario in my panel.

Plus it's just neat.

http://www.trikebuggy.com/products/flytepark/index.htm