Learn Hardcore Korean: riheoseoreseo balseonggwa dongseoneul dasi yeonseuphaesseo. - We practised voice projection and stage movement paths at the rehearsal.

-에서 marks the place where an action happens. So 리허설에서 means at the rehearsal / during rehearsal as the setting where the practicing took place.

Compare:

학교에서 공부했어 = I studied at school
리허설에서 연습했어 = I practiced at the rehearsal

If you used -에 instead, it would sound more like a destination or location rather than the place of activity, and -에서 is the natural choice with an action like 연습하다.

What do 발성 and 동선 mean here?

These are both performance-related words.

발성 = vocalization, voice production, how you project or use your voice
동선 = movement route/blocking, the path or movement pattern on stage

So in a rehearsal context, 발성 is about the voice, and 동선 is about where and how someone moves.

Why is it 발성과 동선을 and not 발성을 동선을?