Indirectly use the [addon=...] special syntax (that makes the SimGear
ResourceManager go through the AddonResourceProvider) in order to specify the
files fed to the 'play-audio-sample' FGCommand. This makes the add-on work
again: since FG commit 8853fded29539593, it wasn't possible anymore to use
'play-audio-sample' with an absolute path (the SimGear ResourceManager may be
changed in the future to allow absolute paths that can be validated for read
access; see the discussion around [1]).
[1] https://sourceforge.net/p/flightgear/mailman/message/37695530/
