I think module-null-sink and module-loopback could be used to create a virtual source which combines multiple sources, though the source/sink thing makes my head spin. Or, more simply, I suppose using the loopback of whatever audio output device does the combination (and the same mixing) for you, if you play all audio through one output device (which is most likely)?

> though the source/sink thing makes my head spin

Wait, what other audio paradigms are there?

Dunno, I meant more that it's an unintuitive way of thinking about a data-flow graph to me, moreso when introducing virtual sinks/sources.