Yes, on my config I use both MP3’s and Nabu Casa’s TTS. I just tried Google TTS and it works as well to Sonos.
I doubt this will make a difference by have you tried just putting the raw internal IP address of your home assistant server address in the call. I gotta say, I really am struggling to see why people call an external service from within their private network to lookup an internal address, but that is just me.
Here is another post, as per this guy, are you 100% sure you have your directory structure correct???
In my setup:
/home/user/homeassistant
maps to
/config
so my physical directory location of:
/home/user/homeassistant/www/sounds
looks like
/config/www/sounds
inside and to Home Assistant
and from a URL standpoint that is:
http://192.168.2.242:8123/local/sounds
https://community.home-assistant.io/t/yet-another-sonos-playing-an-mp3-issue/148307