Is it possible to use IPA within amazon lex utterances?

Hi, you can use <phoneme> tag to improve/modify pronunciation of your texts: https://docs.aws.amazon.com/polly/latest/dg/supportedtags.html#phoneme-tag

Additionally, if it's a matter of audio not being clear enough, please consider increasing volume with <prosody> tag or enabling dynamic range compression with <amazon:effect> (all documented on the page linked above).