why my internal speakers are not working

Go step by step and test.

1. Reset PRAM:  Reset NVRAM or PRAM on your Mac - Apple Support

     Settings that can be stored in NVRAM include sound volume, display resolution, startup-disk selection, time zone, 

     and recent kernel panic information. 

     The settings stored in NVRAM  depend on your Mac and the devices that you're using with your Mac.

2. Reset  SMC:   How to reset the SMC of your Mac - Apple Support   Choose the appropriate method.

    Quote:

    Resetting the system management controller (SMC) can resolve certain issues related to power, battery, fans,

    and other features.

 3. Try suggestions given in this article.

      https://support.apple.com/en-us/HT203186

apple Watch 6 - not working speaker, how to rectify this problem.