[Skooch]

There are two things that are likely to be happening here. The primary issue is as several people have mentioned, fear. Most of us get nervous when we have to speak publicly and it shows in any number of ways. Trying to deal with symptoms (mumbling, rapid speech, etc) isn't particularly useful because often the fear will just manifest itself in some other way. There are a number of techniques people use but I have found the most useful ones center around changing the speakers focus.

The second likely issue has to do with natural speaking habits. If you mumble or talk fast in everyday life you are likely to do the same (only heightened)in public.

Speech therapists and vocal coaches are more likely to help with the second problem than the first. If you can find a vocal coach who works with actors primarily they are more likely to be of help than one who tends to work with singers. I haven't been in the market in some time so I don't know what prices are like these days, but I do know that a lot if it depends on where you live.