Yes, switching your PC to stereo output and feeding that into an external DPL2 decoder will work. I am not sure if DSP HLE will output DPL2 as Dolphin does not have a DPL2 encoder implemented yet, so unless the game uses pre-recorded audio with the DPL2 signal encoded in, that will not work. DSP LLE does emulate the encoder built into the DSP ucode and will work with an external DPL2 decoder.
The DPL2 decoder option is there for PC's which output audio in 5.1 channel, so that the user does not need to switch between stereo and 5.1 between games and Dolphin. It is also for users who do not own external DPL2 decoder hardware.
The DPL2 decoder option is there for PC's which output audio in 5.1 channel, so that the user does not need to switch between stereo and 5.1 between games and Dolphin. It is also for users who do not own external DPL2 decoder hardware.