You might have seen Groovy look like this on your voice channel:
This symbol means that Groovy cannot hear anything in your voice channel.
It does not mean that Groovy is muted. It doesn't have any effect on the functionality of the bot and it can play music even if it is server deafened.
The reason Groovy server deafens itself is to prevent it from receiving your audio.
This significantly reduces Groovy's bandwidth usage (since we don't receive what you say in a voice channel), as well as Discord's bandwidth usage (as they don't need to send it).
Furthermore, it also protects your privacy.