sound output device keeps changing itself constantly

Hi,

Reassure me, you have well a Bluetooth device connected to your (iPad? iMac? whatever 😕), it is not doing that alone? 😁 If yes, it's probably because the connection is lost randomly: the device is a tad too far or the battery of the device needs to be charged, or else some radio wave interfers with the connection (e.g. texting on your phone, microwave oven, etc.).

Regards.

Try resetting NVRAM, and then again select the internal speakers in System Preferences->Sound->Outuput, or from the menubar sound menu.

Start your mac using the power button and immediately press and hold these four keys:

command, option, P, R. Keeping holding until you hear the startup sound some two or three times, then let go.