Advanced Gtk+ Sequencer audio processing engine
https://nongnu.org/gsequencer

Advanced Gtk+ Sequencer audio processing engine is an audio
sequencer application supporting LADPSA, DSSI and Lv2 plugin
format. It can output to Pulseaudio server, JACK audio connection
kit, ALSA and OSS4.

You may add multiple sinks, mix different sources by producing
sound with different sequencers. Further it features a pattern
and piano roll. Additional there is a automation editor to
automate ports.