Describing music in a film does not require specialist vocabulary. You can begin with when it enters, what else is happening and how your attention changes. A precise account of a particular moment often communicates more than a broad claim that the score is powerful or moving.
Keep the description concrete. Does the music arrive before a new image, continue across a change of place or stop during a conversation? If you cannot identify an instrument confidently, describe the quality you hear rather than supplying a technical label that may be wrong.
Next, distinguish observation from interpretation. The music may feel reassuring to you while the scene remains ambiguous. Explaining that tension makes the note more interesting than assuming the soundtrack tells every viewer exactly what to feel.
If you later consult credits or an interview, record the source separately from your first response. This preserves the difference between your experience and verified production information. The two can enrich one another without becoming indistinguishable.