The best open source alternative to Performous is StepMania. If that doesn't suit you, our users have ranked six alternatives to Performous and four of them is open source so hopefully you can find a suitable replacement. Other interesting open source alternatives to Performous are FoFiX (Free, Open Source), UltraStar Deluxe (Free, Open Source) and Vocaluxe (Free, Open Source).
UltraStar Deluxe is a free and open source karaoke game inspired by the Singstar game available on the Playstation. It allows players to sing along with music using microphones in order to score points. It also allows users to create their own songs and sing them.
Vocaluxe is a free and open source singing game, inspired by Ultrastar (Deluxe) project. It allows up to six players to sing along with music using microphones in order to score points, depending on the pitch of the voice and the rhythm of singing.