So I read the Google article here (https://source.android.com/devices/audio/usb.html) on USB audio, and I don't know why your device wouldn't be routing the audio to your DAC. Your device should be in host mode though, and the audio should fit these parameters:
Android 5.0 (API level 21) and above supports a subset of USB audio class 1 (UAC1) features:
The Android device must act as host
The audio format must be PCM (interface type I)
The bit depth must be 16-bits, 24-bits, or 32-bits where 24 bits of useful audio data are left-justified within the most significant bits of the 32-bit word
The sample rate must be either 48, 44.1, 32, 24, 22.05, 16, 12, 11.025, or 8 kHz
The channel count must be 1 (mono) or 2 (stereo)
And further below:
Typical USB host mode audio applications include:
music listening
telephony
instant messaging and voice chat
recording
For all of these applications, Android detects a compatible USB digital audio peripheral, and automatically routes audio playback and capture appropriately, based on the audio policy rules. Stereo content is played on the first two channels of the peripheral.
There are no APIs specific to USB digital audio. For advanced usage, the automatic routing may interfere with applications that are USB-aware.
So according to that, MobileSheetsPro shouldn't have to do anything at all for the audio routing to occur. In fact, if i did try to do something, the automatic routing could interfere with my processing. I think you should focus on trying to see if we can get the USB audio out to work before you completely give up on it.
Android 5.0 (API level 21) and above supports a subset of USB audio class 1 (UAC1) features:
The Android device must act as host
The audio format must be PCM (interface type I)
The bit depth must be 16-bits, 24-bits, or 32-bits where 24 bits of useful audio data are left-justified within the most significant bits of the 32-bit word
The sample rate must be either 48, 44.1, 32, 24, 22.05, 16, 12, 11.025, or 8 kHz
The channel count must be 1 (mono) or 2 (stereo)
And further below:
Typical USB host mode audio applications include:
music listening
telephony
instant messaging and voice chat
recording
For all of these applications, Android detects a compatible USB digital audio peripheral, and automatically routes audio playback and capture appropriately, based on the audio policy rules. Stereo content is played on the first two channels of the peripheral.
There are no APIs specific to USB digital audio. For advanced usage, the automatic routing may interfere with applications that are USB-aware.
So according to that, MobileSheetsPro shouldn't have to do anything at all for the audio routing to occur. In fact, if i did try to do something, the automatic routing could interfere with my processing. I think you should focus on trying to see if we can get the USB audio out to work before you completely give up on it.