I'm not sure what you would do with a STB, and it probably depends upon the STB with respect to whether you'd be able to play them without creating a manual channel.
With a PCI computer card, however it is much easier. I use TSREADER, and it has a window over on the right, which shows thumbprints of any FTA video AND audio.
IE:
The PMT or program map table, is a PID, which lists the A/V PIDs for each program, along with some descriptive info. In the above, I've clicked on the PMT for program 475 at the left, and it shows the A/V PIDs at the upper middle. In the middle, it shows red bars for PIDs that are encrypted, and green bars for those that are FTA. On the right you see little spectrum thumbprints for fta audio streams (some were quiet at the time I captured).
Anyway, with TSREADER, it's easy to tune in a mux, and any fta audio shows up at the right, and if you have VLC installed, you just double click on any of the thumbprints, and it will play the audio.
I had noticed that Nimiq had almost all of the audio from it's HD channels FTA, but I never noticed that it was the AC3 channels that were FTA. In the above example, several of the channels had both MPEG and AC3, and the MPEG audio was encrypted, so that seems to be the case. Interesting.