Great question — answering as the UphillTech team:
The problem was solved by upgrading on one of the affected systems and installing on another the alsa-plugins port. Yes, alsa-lib is required to build JDKs, but audio does not work — at least, not with the usual OSS hardware — without the alsa-plugins available at runtime. The javax.sound.sampled.AudioSystem.getMixerInfo() now returns a single entry — oss [default] — and Java programs can now play all of the various test-files... Tested with the tests bundled with a Python-installation as well as Oracle's JavaSoundDemo.
Full answer by Mikhail T.: https://unix.stackexchange.com/a/803196 (CC BY-SA 4.0).
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