take care what module you buy, where are version which did not have support for all features,read description of what you buy, for example here from aliexpress:

This module at aliexpress works with my breadboard adapter, at least one I buy few months back.

This one has only BT and AUX funtionality, it make poping noise, then module is switching between internaly generated sounds and BT or while swtching from BT to AUX, probably need to use mute pin for muting poweramp, buttons areon ADC, in this PDF: https://github.com/tomaskovacik/kicad-library/blob/master/library/datasheet/F-6888_bk3254/F-6888_BK3254_datasheet.pdf. I need to state that module which I receive hase these markings: F-6888, V1.2 2016-8-6, so this is not indication of anything!, shame :(

Supported commands based on this datasheet translate by www.onlinedoctranslator.com:

https://github.com/tomaskovacik/kicad-library/blob/master/library/datasheet/F-6888_bk3254/BK3254_V1.2_HYT.zh-CN.en.pdf

and this:

https://github.com/tomaskovacik/kicad-library/blob/master/library/datasheet/F-6888_bk3254/BK3254_V1.3_HYT.zh-CN.en.pdf

Follow this link to arduino.cc: https://www.arduino.cc/en/Guide/Libraries#toc5

this is for testing if your module has serial interface enabled:

For supported commands and events chek this wiki.

For supported functions chek this wiki or check example.

On this this wiki page is list and variables which are set by librarie when event is received from module.