Sometimes, you wish to describe or write out something that may be difficult to put into words. They could be the sound a person makes when getting punched, or the sound of water crashing onto rocks. There are two methods by which this can be accomplished; first is to make simplified Onomatopoeia (on-a-mon-a-pee-a); Onomatopoeia basically means to describe the sound. Examples of this include clang, boom, pow and thwack; words not used for their accuracy, not to instill the kind of sound a person associates with what you are looking for.
EX: Boom! The explosion could be heard from miles away, as it rang in the ears of those close by.

Onomatopoeia can also be used in sentences; ugh is used when a person is annoyed or hurt, hmmm implies deep in thought, and ack is an indicator of surprise. These are normally one shot sentences; this means that you do not say anything else within that grouping.
EX: "Ack!" She exclaimed, surprised by her friend. "You scared me."

The second method is the tried and true description. Take for example when a character is muttering; their voice is too low to say what they are trying to express, but you can't simply pass it over and pretend it isn't happening. Describing what a person is doing that help accomplish this.
EX: The man spoke lowly, but with a tone of annoyance; though his mutterings were too incoherant to make out, it was clear he was not happy