When Miriam sang would the occasion have had more in common with a formal worship service or a youth bonfire at the beach? I'm curious as to why there's any question about it.
You have to remember that we have a new pattern for formal and gathered worship that is simple, spiritual and scriptural in the New Covenant era i.e. unaccompanied Psalm-singing.
This is appropriate to the more mature New Testament church that does not need soloists, musical intruments and other fancy accoutrements in order to worship God.
So simple, in fact, that it contains no elements to which anyone who accepts the Bible as an authority could reasonably object. The fact that we even have to debate solos is almost enough to establish that they are not commanded. Let's say we introduced a special solo into my church's service. All the other elements of our worship can be easily and quickly proven from Scripture as required and practiced by the Apostolic church(reading and preaching of Scripture, singing of psalms, prayer, the Lord's supper, etc.). So when someone asks us why this element has been introduced, what do I tell them? "The prophetess Miriam once after a miraculous event led a group of women to celebrate with dancing and tambourines and an inspired song she wrote; therefore we can have solos in NT worship (but not the dancing and tambourines; what are we, charismatic?)" I don't think it's a credible proof.