is it possible to play a wav file to all conference members using ALTER CALL SET_INPUT FILE= ??
Yes or by Port, however, the opposite is NOT true, you can only record the OUTPUT of one of the callers in a conference call as the host.
You can only be using one INPUT, one OUTPUT and one CAPTURE_MIC at a time, you can't be doing two of any of these with different calls simultaneously. More here:
https://developer.skype.com/Docs/ApiDoc/Voice_Streams