You will not be asked to transcribe a live audio sample. Praxis 5331 transcription items are printed: the stem gives you a target word and a child's production written in IPA, and you decide what process or error the transcription represents, or you pick the transcription that matches a described production. That means the skill being scored is reading IPA accurately and mapping symbols to articulatory descriptions — not fast phonetic ear-training.
Because of that, the highest-return preparation is symbol fluency for the sounds that show up in disordered speech: the interdentals /θ ð/, the postalveolars /ʃ ʒ tʃ dʒ/, the velar nasal /ŋ/, the glottal stop /ʔ/, the rhotic vowels /ɚ ɝ/, and the lax vowels /ɪ ɛ æ ʊ ʌ/. Nearly every transcription item on the exam uses one of these, because they are the sounds children distort, substitute, or delete.
Diacritics appear less often, but two are worth knowing cold: the nasalization tilde over a vowel (hypernasal resonance items) and the dentalization bridge (frontal lisp items). If a stem shows a diacritic, it is usually the entire point of the question.