Yes, this is quite correct for AGA Amigas, it automatically senses if you only have one RCA plug connected, and provides a mono output.
Anyone doubting this are quite welcome to try themselves, fire up a good old mod while unplugging one of the RCA plugs.
And for those of you with PicassoIV's:
The P-IV gets the Amiga audio from the Video Slot, and passes it through to the output jack of the P-IV (or Concierto if installed).
On my A4000, this was mono until I plugged to empty/cutoff RCA jacks into the mobo audio output connectors. A side effect of this feature...
-Paul
WTB:Grex PCI for A4000D