sox? awesome! Can we create some effect like "robotic voice" or "female voice" with it?

The adapter I created doesn't support such effects.Actually, it's out of domain of that gem. But, I've created a ruby wrapper for SoX, you can use just it to apply any sox effect from ruby: https://github.com/TMXCredit/ruby-sox . So, sure, you can combine AudioGlue and ruby-sox to achieve your goals)

Only mp3? Are there other formats?

No. It can be everything. The main thing is to implement adapter which would support all formats that you need. We've implemented an adapter based on SoX unix tool which supports pretty many formats: https://github.com/TMXCredit/audio_glue-sox_adapter

fredwu, thanks for Datamappify! Looks interesting.

Thanks @happypeter .