Your comments

the problem was solved with the last firmware update by HTC (2.50.405.2)

I was thinking the same thing. Establish a time beyond which the streaming is stopped. So if a call is short I'll be back to my music quickly, but if the call is long, I save the data.

For my use would be fine 30 seconds.
It could become an advanced option of the application.
Yes! The problem occurs even play MP3 from SD memory, and watching video from YouTube (via bluetooth A2DP). It works correctly if I listen to music from the speaker or wire headphone.
The issue seems not to affect voice calls via the bluetooth.

I have Xiia Live 2.1.8 moved to SD, and i can't find the widget in the list.
The only way to see Xiia widget in my widget list is move back the app to the phone ?