I saw someone talk about a 「はあっ」that they saw in a book, so I looked it up but didn't find much, just some words/sites that had あっ in them. (I couldn't read the rest) I was wondering how one might pronounce っ at the end of a word. I'm also not sure if っ can be applied after あ only, or other vowels/characters.
Let's look at the term 学校 for example. This is equivalent to がっこう which is pronounced as gakkou meaning school.
Because of the small つ, namely っ, then the romaji is spelt with two letters. Since っ occurs before こ then the two letters in the romaji spelling would be k.
Thus, how must we pronounce the term はあっ if there is no letter following from っ?
It is a good question, and the answer is that we pronounce this as ha as opposed to haa.
ha has an abrupt ending off the a vowel (short vowel sound), and haa has a long vowel sound at the end. However, we could simply write は, right?
Well, let's consider the following conversation:
"Hey, Jimmy! How are you to—"
"Shut up, Tommy!"
The person who spoke second had cut the first person speaking off. If we were to write this conversation in Japanese, instead of using the hyphon, —, we would use っ instead.
So if someone said 「は」then this would just show that one has said the statement completely and finished it off, without ever being cut off in the process. But, to show that someone was saying a sentence or a word that began with は but was abruptly cut off, we show this by writing「はっ」instead.
But because it is written as 「はあっ」 with the letter あ, then this shows that whoever was speaking and was cut off, he/she was going to say a word (or begin a sentence) that began with はあ in particular.
Go here for another reference. The comment below your post has also provided the same link.
When っ is at the end of a word, it indicates a glottal stop or a “choked sound” according to Wiktionary. Imagine that you suddenly remembered something and went “Ah!—” That would be close to how あっ would sound.