Are there any modern Linux distributions that still support /dev/audio?

If your computer uses PulseAudio (Ubuntu does), run the program via padsp:

padsp yourprogram

It will intercept attempts to open /dev/audio, /dev/dsp, and other related devices (using a LD_PRELOAD shared library) and send the sound directly to PulseAudio.

For systems that use plain ALSA, the equivalent is aoss from the "alsa-oss" package.

aoss yourprogram

aoss should work on Ubuntu too, since by default ALSA itself is rerouted through PulseAudio, but better use padsp in that case.

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Most kernels also have the snd-pcm-oss module, which provides real /dev/dsp and /dev/audio devices using ALSA:

sudo modprobe snd-pcm-oss
yourprogram

I've heard it doesn't work as good as aoss, though, and I'm not sure whether it works at all when PulseAudio is running.