In classical mythology, the Muses were viewed primarily as musicians, often performing for the gods, and were strongly associated with their mother, Mnemosyne (Memory).
One interpretative answer to the original question is that the Muses have helped (“oftentimes beene to me ayding”) Spenser to write “unhappy songs” in the past by playing their instruments (“Your string could soone to sadder tenor turne /And teach the woods and waters to lament . . .”). Their music aided his memory of sorrowful events and provided a harmony for him to emulate in verse.
The questioner may be referring to Spenser’s Epithalamion.
As stated above, Spenser often invoked the Muses at the beginning of his poems. In addition, an invocation of the Muses is a typical element of the epithalamion, a wedding poem or song. In Epithalamion, the poet asks the Muses to assist in his happy task of describing his bride. He notes that the Muses have helped him write “unhappy songs” (as the questioner puts it) in the past:
Ye learned sisters which haue oftentimes
beene to me ayding, others to adorne:
Whom ye thought worthy of your gracefull rymes,
That euen the greatest did not greatly scorne
To heare theyr names sung in your simple layes,
But ioyed in theyr prayse.
And when ye list your owne mishaps to mourne,
Which death, or loue, or fortunes wreck did rayse,
Your string could soone to sadder tenor [mood] turne,
And teach the woods and waters to lament
Your dolefull dreriment [sorrow].
I think you are right, it's certainly this but I still can't see how the question could be conveniently answered. The Muses are there as mere ornaments "to adorne".
I think that a writer, especially a poet, calls on the muses to try to grasp something of the grandeur and the beauty of the "golden" past whether to mourn or to praise. A muse stands for that powerful tradition, that remote and exotic ideal of a quintessential and absolut form of expression whether sad or jolly.
I have not read much Spenser but in this I think I may help. Spenser like most poets following the classical tradition invokes the muses. However, 'muse' does not necessarily mean 'joy' or 'bliss' The Muses in question are nine and not all of them inspire happy songs. For instance, Thalia is the muse of comedy while Melpomene is the muse of tragedy. The difference is quite salient. A muse inspires and the inspiration it grants to the speaker is that of being able to voice his emotions and passions. It may well be happiness but it can also be pain. I think the best example epitomizing this pardoxical role of the muses, especially the role of Erato the muse of love, is to be found in Sappho. One poem of this great Greek poetess survives in Longinus's work On The Sublime. Have a look at it.