Basically when the audio signal is converted into air particles (basically sound waves) with your speakers. They will travel through the air. Depending to their frequencies, these waves are going to be completed in different places in your room.

Like a 20Hz. wave requires ~17.3 meters to be completed in 25 celsius degrees. and a 20Khz. wave requires 1.73 centimeters to be completed. (in 25 celsius degrees) Which means : You can actually hear 20Khz. of audio 1,73 cm. away from your speakers. But you will need to go 17 meters away from your speakers to actually hear the 20Hz.

Why you can’t subtract the internal audio , from the external audio :
The microphone will be standing somewhere in the room. And it can’t simply get all the frequencies because of the reason I told you about. In a song, there are hell a lot of frequencies, mixed, and finalized in one audio file. And because of the differences of wavelengths of frequencies, phase differences, the microphone will not be hearing the same exact output thats theoretically coming out from your computer. (there are many more reasons for this, like objects reflecting sounds, souring frequencies, reflecting selective frequencies, the room resonance and etc. ) The microphone will get the music! yes… but the audio it’s getting, and the final frequencies of the music will be completely different from what should be outputted through your computer.

Now, yes you can make the computer calculate the rooms’ acoustic parameters. We use a software named : ODEON Acoustics to design acoustical environments, to predict the acoustical result of the environment, and for the auralisation of the acoustics. { http://www.odeon.dk/ } With this software, we completely design the room / conference hall / stages / concert halls etc. and we place every single detail into the environment. Ex : If this environment is a cinema, we place the comfort chairs, speakers(transmitters), we input the technical details of the speakers, even what kind of cloth the seats use… (the materials of every object in that acoustical environment) Then, you set a receiver point. A coordinate in the room, or even outside the room. I mean a coordinate. (a microphone or a listener {you can add multiple receivers too} ) You give a *.wav file of the estimated audio, that’s going to be heard inside. And the program predicts, how it is going to be heard. Ex : you select 5A in the cinema, and give a *.wav file of the audio. Then you put on your headphones and listen from the ears of the seat 5A

This program is a gigantic program, and takes a little bit of time to calculate all the parameters. The most important thing is you have to design the environment, or you have to load a cad file (from AutoCad or stg.)..

Why you can’t subtract the audio from the voice , because , you can’t get a calculation without the room parameters
These parameters are different for every single room on this planet.
So you can’t make a universal calculator. You HAVE to input the room parameters.

You can make something, that subtracts your voice out of the music. but your voice will probably get lost, or literally “blurred”…

And Voice Recognition systems, calculate the wave and frequency differences of the words and the phrases. So they can only resolve your voice, if the frequencies are sharp, and understandably “unblurred”…

The only way to do what you want is , to use a simple Bluetooth headset, and an iPhone.

You’ll use the iPhone remote application to lower the volume of the music, and then give your commands with your bluetooth headset microphone

Hope I could clarify some of the main problems for you.
If you need any help, feel free to e-mail me.

cheater{dot}boss{at}gmail{dot}com