1. I search for a function to get all possible speakers from a soundcart.
Do you want to get the number of speakers? If so, that is available via BASS_GetInfo (see the "speakers" value).
2. In other software, the user can select "Speaker Front" oder "Speaker Front Left". I use the function Bass_Speaker_Pair1 2,3,4 etc... my question: where is the link between this?
You can use BASS_SPEAKER_FRONT for the front pair of speakers, or BASS_SPEAKER_FRONTLEFT for the front-left speaker. Please see the "Speaker assignment flags" section in the documentation for details on the available speaker options.
3. I use the function "bass_StreamCreateFile...." in future I will use Asio. Can I integrate Asio in my exist Code?
Yes, you can support both normal BASS/DirectSound output and ASIO output (via BASSASIO), but they will require some different code paths, eg. via some "if" statements. When using ASIO output, you would need to add the BASS_STREAM_DECODE flag to your BASS_StreamCreateFile call, and use BASS_ChannelGetData to decode data from the file in the ASIOPROC callback function. Some examples of using BASS processing with ASIO output can be found the BASSASIO package.
4. Is there an function to activate the playback from a microphone? I try it with Windows API, but with Windows 7 it doesn't work.
I'm not entirely sure what you mean. Can you please elaborate?