This is an OS issue, not Mojo. But there are ways around it - I got this from Chord's Matt:
It should be possible to listen to a YouTube clip and have a music player program such as J River Media Centre running at the same time.
What you need to do is go into the Windows Sound Properties then go to the advanced tab and find the Exclusive mode option.
Make sure this is unticked then apply the settings. Once this is done close everything down and
reboot your computer.
After the reboot you will be able to keep programs open and play from them as you like.
You can only play music from one application at a time so you must pause or stop playing before playing from another program.
This works with ASIO.
Alternatively you can just use direct sound mode which will pass all audio through the Windows Mixer so everything will play but then you have to be at
a fixed sample frequency.
There is a good explanation here on the J River Wiki site.
https://wiki.jriver.com/index.php/Exclusive_Access
Feel free to try this to see if it works - I can't guarantee it will work with all programs but it is worth a test.