The conventional color code used by MOST custom integrators and most installers is:
Right channel- Red is +, Black is -
Left channel- White is +, Green is -
This is typical for in-wall rated speaker cable- it may have some kind of marking on the jacket- if you can read (or photograph) this, maybe you can post it. It will have a brand (Honeywell, Coleman, West Penn, Belden, etc), part number (Honeywell 5254 or something else), CL rating, foot marker and a series of letters and numbers, which are used for designating the cable to where it goes. Many people use labels, instead.
Red/White has been used for RCA jacks to mark the Right/Left channels for decades- they wouldn't change that for speaker wire markings.
"4.3.2
Speaker Wire and Connector Color Codes
All speaker cable conductors should be color coded, or distinctively marked to ensure proper polarity.
Two conductor cables generally contain a red and a black insulated conductor. The red conductor
shall be used for the positive terminals and the black conductor shall be used for the negative
terminals. Four conductor-unpaired cables generally contain red, black, white, and green conductors.
Red and black shall be positive and negative respectively, for the right speaker in each zone. White shall be used for positive and green shall be used for negative to connect the
left speaker. Other colors may be used, but all connections must be consistent.
Speaker terminals shall be color coded in accordance with the color-coding scheme in CEA-863-A.
Special considerations should be taken for active speakers; follow manufacturer‘s instructions."
Turn the volume control fully clockwise and connect one of the wires in the red/black pair to a AA battery and tap the other wire on the other end- have someone go to the area that may be marked on the cable to listen- they should hear a scratchy tapping sound. Repeat this with the other pairs. If you don't hear anything, try holding one wire on the end of the battery and tap each of the other wires on the other battery terminal- it's possible that whoever installed it didn't use the conventional colors. If they hear nothing, make sure the speakers are still there.
I wouldn't use Bluetooth if this bedroom is far from where you want the music to play- I would recommend something that uses WiFi because the range is far greater.