For audio streaming, the drive is told by the game to play stream xy from offset z. And then the drive reads the data and directly sends it to the audio processor. When you load games from sd card, sd gecko or backup disc, you have 0 control of the dvd drive, so the dvd drive can't send stuff to the audio processor. For WODE and any other chip that replaces the drive, it's technically possible to do the audio streaming, but the WODE team had to decide beween ripping retail discs and audio streaming, or something like that...
You could play the audio streams as sounds via the ppc, but for this you would need extra processing power, additional memory, a thread on the ppc(!), code to convert audio streams to playble formats and most likely something else i don't even begin to understand. Before people start bugging about this and spam the thread: If you really want to talk about this: Do it in another thread.