Would you recommend diazepam/Valium 10mg for public speaking/speeches?

I agree with the two other responses- ask your Dr. for a beta blocker.
Valium has a very long half life and you could find yourself sedated for the entire day and into the next day.
A beta blocker will keep your nerves in check and you shouldn’t feel too sedated. You can even take a half of one to see how it affects you. Personally I find them to be superior to benzodiazepines at controlling anxiety.

Do you find your heart rate rise and beat fast? Because if so the previous suggestion for a beta blocker might be worth trying. I’m immune to benzodiazepines, so I found my own way via trial and error.

I have had the same voice cracking and almost unable to get the syllables out of my throat.. the unfortunate thing is that I excell and enjoy public speaking but I found much had to do with the audience and setting. I found walking around speaking area very helpful while standing still behind a podium caused my anxiety to worsen.
Best of luck.

Diazepam is one choice but 10mg for someone to take as needed is quite a lot and likely to cause you to go to sleep. For a Benzo naive person I would suggest something more in line with 2mg and even that might be too much. For public speaking issues a lot of people find Beta blockers quite effective. Talk to your doctor about it.

Take care.