Which one?

[PlacidRan]

"M--may I help you, sir?"

or

"M--May I help you, sir?"

Also, would a hyphen work instead of a dash (just out of curiosity)?

Example: "M-May I help you, sir?"

[tedtmc]

"M--may I help you, sir?"

or

"M--May I help you, sir?"

Also, would a hyphen work instead of a dash (just out of curiosity)?

Example: "M-May I help you, sir?"

What is the 'M' doing in front of the sentences?

not a teacher

[chester_100]

M--may I help you, sir?

or

M--May I help you, sir?


I think that's used to show hesitation. By capitalizing the first letter, you've started the sentence, and it's not necessary to capitalize the first letter after a hyphen. But I'm not sure if it's ok to use two hyphens that way.
Structurally, the first part is an interjection. Here's an example:
-Aha, you did yourself!
-M-m-m, may I help you sir?


Also, would a hyphen work instead of a dash (just out of curiosity)?

Example: M-May I help you, sir?
Of course not. Hyphens are exclusively used for morphological divisions (compounding), not for sentential ones.

C

[tedtmc]

-M-m-m, may I help you sir?

Aha, I see what what you mean now.
But I don't think you can have this in written English.
Double hyphen is also unheard of.

[PlacidRan]

That's not supposed to be a double hyphen, but thanks anyway.

[chester_100]

Aha, I see what what you mean now.
But I don't think you can have this in written English.
Double hyphen is also unheard of.

How about:


-Mmm
Or
-Hmm

[tedtmc]

How about:


-Mmm
Or
-Hmm

Yes, those interjections. They are accepted words in Scrabble.