For any USB DAC, the user has to choose between whether to give the DAC an exclusive access by a particular program (e.g. JRiver) or allow multiple programs to output sound at the same time.
Giving exclusive access means the program can directly send the digital data (bit perfect) to the DAC without compromise, so most audiophile programs like JRiver default to this method.
When you install Mojo's drivers onto Windows, it actually installs three different drivers. DirectSound, ASIO and WASAPI. Windows can only see DirectSound, whereas JRiver can see all three, and you can choose which one to use. Generally ASIO or WASAPI are preferred because they are designed to give exclusive access to the DAC.
DirectSound is what Windows uses (this is what shows up on Windows Sound Properties). It isn't preferred for audiophile listening because it is designed to send the music to Windows Mixer, which allows multiple programs to output the sound at the same time. Because each program may be using different bit depths and sample rates, Windows Volume Mixer's job is to make sure they are all converted and mixed to the sample rate specified in Windows Sound Properties (e.g. 44.1kHz). All sound is converted to 32 bit float then dithered back to 16 or 24 bits, so it is not bit perfect.
Old Windows Mixer on Windows XP were horrible so people historically stay away from DirectSound. Mixer on Windows 7 and up are pretty decent, but people still prefer exclusive access via ASIO/WASAPI drivers, because we want to ensure pure playback and avoid random noise (e.g. e-mail notification sound effect etc.) to interfere with music.
The bottom line is, you have to choose between absolute purity vs. convenience. You can set up to have multiple programs to output to Mojo at the same time, but JRiver is not set up to do that by default, so you need to change the settings.
