Get sound working on more CI platforms

[ssfrr]

The main issue is that there aren't any audio devices on the Travis machines, so we can't even try reading and writing. Even if we could, there's no way to simulate input or output, so we can't really check that the right data is going in.

One option would be to put together a fake libportaudio with the same API but some additional mocking functionality. We'd only have to fake the subset of the library that we're actually using.

[bramtayl]

We have CI on mac since #65, but it would be nice to get it working on more platforms.