Student or Learner
" I could rewinde this tape 15 minutes beforehand/before"
'Beforehand' would only refer to something happening before something else happening at a moment in time.
The word 'before' can be used in more situations. For example as a preposition of place. (He stood before me)
In your example sentence I think both words work but 'beforehand' is more proper, I think it gives a feeling of preparation and readiness.
(The spelling is 'rewind'.)