This is a bit of a long shot, but a lot my social embarrassment comes from my tendency to mumble / be hard to understand. Anyone on here have any experience with improving at that stuff?

Take a look at Vinh Giang [1], he specializes in communication skills and vocal clarity, and is a super fun guy to listen to speak.

[1]: https://www.youtube.com/@askvinh

On the practical side there are many good options. Using the voice effectively is a key skill in many professions, such as singing, public speaking, education, religious, management. So there are many practitioners, and lots of learning materials out there. So I recommend to check of there are any of those angles that resonate more with you. But very important - this is a challenge that needs practice to overcome - it cannot be dealt with only theoretically! So as a simple starting point is: go on YouTube and search for vocal exercises. Start with something like warmup routines. And then try something like saying the same sentence at 7 different loudness levels, from a whisper into someones ear to a SHOUT that needs to go to another person at the other side of a lake. Observe just how much power you got! This will be awkward and that is good - it means that you are stretching yourself, both physically and mentally! Like at the gym.

Then out a few sentences in the level you feel most comfortable with, plus 1 or 2 steps above. Try whole paragraphs.

PS: drink water and do not push harder if you feel pain.

Studied Film and worked professionally in overdubbing for a while. I recommend in that order:

1. figure out if your mumbling is just a habit or a logopedic issue

2. get a speakers training of the type actors or radio hosts would

If you wonder whether it is worth it, just consider how important it is in both your private and professional life to be clearly understood. There has been a study during the pandemic that showed that people who had the better sound quality in conferences would get their ideas through more often. This is truly the case for spoken words as well.

Not an expert so take this with a HUGE grain of salt.

I think that the answer might depend on whether the difficulty is something with your speaking itself (e.g. you have a speech impediment, accent, stutter, etc.) versus if the difficulty is with the social side of things (e.g. mumbling due to nervousness, etc.).

For the former, maybe a speech therapist could help? For the later, I've heard people say that improv classes/clubs can do a lot for helping people be more confortable in unpredictable social situations.

This may sound unhelpful, but just try to notice when you do it and then speak more clearly. Try to minimize the amount of "What" as a response to what you say.

If you have a medical type of problem that's of course not applicable.