@PotomacBobI would use the hyphens, especially since this is dialogue.
That said, it's worth mentioning that I'm a purist when it comes to anything inside of quotes. By that I mean that you are trying to replicate, as closely as possible, the actual sounds being made. Dialogue should not include any abbreviations or contractions except where that form is what is being said.
So, for example, Mr. Smith should be written as "Mister Smith" in dialogue. Even though Mr. is a standard abbreviation that almost any English speaker would know, in dialogue it is important to indicate the sounds, not the writing conventions.
So from that point of view I think that "Mo-om" or "Mo-o-om" does a better job of indicating actual sound than "Mooom" does, which could also be read as rhyming with "zoom".