I had a similar problem and discovered that the internal speakers would no longer work if I had 'Enhance Audio' switched on.  Go to 'System' -> 'Sound' -> 'Properties' (or Speakers, Intel Smart Sound Technology for MIPI SoundWire Audio etc) and you should see an option for 'Enhance Audio'. Switch this off and hopefully, your internal speakers will work again.  Strangely if 'Enhance Audio' was on, I could hear sound from Bluetooth headphones or external speakers without any problems.
I just got my new XPS 13 Plus laptop in the mail a couple days ago. Everything was great on the computer until I ran into a sound problem. I connected my bluetooth headphones to the computer. No problems occurred when connecting and listening with my headphones. The problem is when I disconnected the headphones and now I do not have any sound. I would consider my self as a pretty tech savy person, so I went through the process of troubleshooting and could not find a solution. I read some other forums on the Dell website with the same issue but not solution.
XPS 13 plus sound not working