Does the DM13 have a built-in battery?

No. It is powered over USB, so you need a USB power source or adapter to use it. That also means no charging wait, but it is tethered while playing.

Can it play files from a microSD card or stream?

No. The DM13 is a CD player only. Its USB port is for power and for ripping or recording discs to a storage device, not for playing files back.

What can I connect to the 4.4mm output?

The 4.4mm jack works as a balanced headphone output and as a balanced line output. Use it for balanced headphones or IEMs, or feed a balanced input on an amp or active speaker.

How does USB ripping work?

Connect a USB storage device and the player saves CD tracks as WAV files. You then move them to a computer or phone with software for playback or sharing.

Can I use it as a source for an external DAC?

Yes. It has optical and coaxial digital outputs, so you can send the disc's digital signal to a separate DAC or receiver and use the DM13 as a transport.

Will it drive high-impedance headphones?

Fiio rates the balanced output at up to 660mW, which covers most earbuds and many full-size headphones. Very demanding planar or high-impedance models may still benefit from a separate amp.