Right now my HT setup is a Mac Mini connected to my 3 or 4 year old Sony 5.1 500 watt reciever. The reciever has mode buttons so you can change between 2 Channel Stereo and 5.1. I am willing to bet that if I downloaded the manual and looked, I could figure out a way that it would be able to set preferences per input (maybe?)

However I need it to be able to output 5.1 and 2 channel on the same input - the Toslink optical input from the Mini.

I watch movies and TV shows from the Mini in which case it needs 5.1 mode so that the center channel is used for speaking audio.

But I also listen to music and I dont want to have to get up and change it to 2 channel mode each time. If I try and listen to music from Itunes while its in 5.1 mode, it plays with most of the audio from the center channel and I hear some faint things coming from the L/R speaker. If I press the 2 Channel button, the music sounds as it should (or at least how I think it should).

Are there recievers that can automatically decide to play the input in 5.1 or in 2 channel stereo? Or is this something I need to do on my computer, because its optical out and maybe if I change the signal through an applescript somehow to make it output in 2 Channel only, then the reciever will automatically do that.

I know very little about this stuff, I am very much a newbie and I apologize if I used improper terminology. I know the reciever has many many modes and I only ever kept it on the default mode - and for the past 4 years thats been fine since I have been using the crappy satellite speakers it came with and I just "invested" $550 on Primus 250s for L/R/C and I want to make sure I am maximizing my "investment".

Any avr can play 2 channel music and dolby prologic from a 2 channel source and since you are feeding the avr a 2 channel signal it will not play 5.1 discreet ,because to achieve that you will need a coaxial digital or optical digital signal and not a 2 channel miniplug from your computer's sound card.

The Mini has a 3.5mm jack audio output, but its optical output.

I am sending an optical audio signal to my reciever as described above.

Normally the reciever takes that optical input and does 5.1 but even when I play an MP3 from ITunes, it does 5.1.

If I can figure out a way for ITunes to force the optical output to go at 2.1 then maybe the reciever will automatically detect that and change it to 2 channel mode? Or is it something that a more advanced reciever can do on its own, or do I just have to go through the hassle of pressing the 2CH button each time I want to change from movies to music?

iTunes on your Mac does not output 5.1 on stereo mp3/AAC. Your receiver is set to convert all pcm audio (stereo) to 5.1 for that input. If you post the model number, perhaps we can download the manual from Sony to find the answer.