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audio: Add amsynth.

amsynth is an easy-to-use software synth with a classic subtractive
synthesizer topology.

- Dual oscillators (sine / saw / square / noise) with hard sync
- 12/24 dB/oct resonant filter (low-pass / high-pass / band-pass / notch)
- Mono / poly / legato keyboard modes
- Dual ADSR envelope generators (filter & amplitude)
- LFO which can modulate the oscillators, filter, and amplitude
- Distortion and reverb