USB

Voices on the internet says that Android supports USB audio.

Others say it don't.

This is what Android is saying.

Accessory mode

Android 4.1 (API level 16) added limited support for audio playback to the host. While in accessory mode, Android automatically routes its audio output to USB. That is, the Android device serves as a data source to the host, for example a dock.

Accessory mode audio has these features:

Accessory mode audio has not been widely adopted, and is not recommended for new designs.

Host mode

Android 5.0 (API level 21) and above supports a subset of USB audio class 1 (UAC1) features:

Host mode is USB audio as we know it when connectiing the DAC to a PC.

The PC (or in this case the phone) is the host.

Powering the DAC might be a problem.

This can be solved by using a powered hub.

 

Most Android devices have a micro-USB connector that does not permit host operation, an on-the-go (OTG) cable is required.

OTG

USB Audio Tester

A little app by Daniel Sobe available on Google Play

 

Select the USB device, in this example my Benchmark DAC 1 USB

 

All the sample rates supported by the USB receiver of the DAC are listed.

 

Nothing will play!

 

This is Android 5.0.2 on a Samsung Alpha.

Obvious 24 bit is not supported.

Third party

USB Audio Player PRO has its own implementation of USB audio.

References
  1. USB audio - Android Open Source Project
  2. PulseAudio vs. AudioFlinger - Arun Raghavan