not getting usb serial on macbook

When I connect usb to the dfplayer pro, it says “slave mode” over the attached speakers.  I don't have any usb serial devices listed through the command:

`ls /dev/{tty,cu}.*`

I am able to see the device as a flash drive in the finder though and copy audio files to it.

Is there some need to move to “AT command” mode to get a serial connection?  If so, how do I do that?  I am only powering the dfplayer pro through the usb cable at the moment.

The play button does work btw to play the audio files also.

The main reason I want this is to turn off the default factory audio per the FAQ