Hi lynoo,
You can just connect the USB sound card and it should show up as an additional audio output in your advanced info’s sound card selection of Squeezelite and Shairport. We have successfully tested the LogiLink 7.1 card and the XFi by Creative, but generally most cards should work with the driver. Regarding Jivelite, it is just the graphic interface for your Squeezelite player, so depending on which output you select, the music will play through the corresponding device. You can add another instance of Squeezelite with the MultiSqueeze plugin to run both USB sound card and digipi+ simultaneously.
Thanks a lot for your answer!!
I did manage to select and use an additional audio output. I have not tried the multisqueeze option so far, but I will!
Do you know if there is a way to optimise the USB output for audio signal? I have already overclocked the RPI and increased the output current intensity in „settings“ (i dont know if it changes any thing 🙂 )?