I've been listening to some Cantonese audio from Pimsleur and noticed they use a different word for "what" compared to what I learned from CantoneseClass101. In CC101, they used 咩, such as "咩事啊?", whereas the audio I just listened to is using sentences like "你想飲乜野啊?" What is the difference if any?
It's a contraction. (合音)
Cantonese contracts 乜嘢 (mat1 ye5) 'what?' to 咩 (me1).
Common words spoken fast often have the first initial mixed in with the last final. For example 咩 takes the "m" from 乜 and the ending "e" from 嘢, creating the contracted form: me.
Same meaning, same usage - just quicker speeds.
You will find many contractions in Chinese topolects.