Android still doesn't handle USB audio very well. USB DAC dongles have their own volume settings, however Android doesn't ever change this. This means that if a dongle has a preset volume level that is relatively low, android can never go above this.
Help us get this issue higher up on android's priority list by pressing the +1 button. I know it probably doesn't affect many users here but thank you anyway!
My car doesn't have Bluetooth, and my phone doesn't have an aux jack, so I am all too familiar with this problem. My ex with an iPhone used a dongle-to-aux which worked fine, but my phone was always incredibly quiet, so basically unusable. It's a shame that Google isn't really doing anything about this problem. I did my part and starred it, looks like there's only ~200 stars at this time
Classic Google, mocks apple for removing the aux port. They then remove the aux port and don't even bother implementing the aux dongle protocol correctly.