I have a dream with spectacular orchestral music reverberating
ご質問ありがとうございます。
「壮大な」はresplendentとかspectacularとかamazingとかbeautifulなどに訳せます。
例文:Just as I am falling asleep, I often have a dream, filled with resplendent orchestral music. It's less a dream and more of a healthy hallucination, I think.
ご参考になれば幸いです。