From the ALSA wiki
Ecasound is a software package designed for multitrack audio processing. It can be used for simple tasks like audio playback, recording and format conversions, as well as for multitrack effect processing, mixing, recording and signal recycling. Ecasound supports a wide range of audio inputs, outputs and effect algorithms. Effects and audio objects can be combined in various ways, and their parameters can be controlled by operator objects like oscillators and MIDI-CCs.
Ecasound's homepage is at from http://www.eca.cx/ecasound/ and the above description is taken from there.
A handy magic incantation for testing if jackd and ALSA are working might be:
jackd -d alsa -d via -s & \ ecasound -f s16_le,2,48000 -i XXX.ogg -o jack_alsa jackd -d alsa -d card0 -s & \ ecasound -i resample,auto,XXX.ogg -o jack_alsa
You'll need to have a test sound file for it to play. In the above example this is XXX.ogg.