Reply To: Rosh Hashana davening in BP

I read once that singing in Tefillah has its roots dating back to the Bais Hamikdash. It’s a zecher, an echo of the song the Leveim used to sing daily in service for HaShem in the Bais Hamikdash. That’s why on Shabbosim etc. when the ‘olam’ generally has more time, we express ourselves in song to our Creator at parts of the Tefillah.