Evolving weapons happen by combining a max level weapon with any level of a specific secondary support item. Once you have both of those items you must then kill an Elite enemy and open the chest that it drops. The combinations for the Song of Mana is the following:
Mannajja – Song of Mana + Skull O'Maniac
This will be one of the later evolutions that you'll come across as each of the items has a pre-requisite before you can unlock it.
Song of Mana is unlocked by unlocked by surviving 15 minutes with Poppea, which is a character that is unlocked by finding the coffin in Dairy Plant. Skull O'Maniac is unlocked by surviving 30 minutes with Lama, which is a character that is unlocked by surviving 20 minutes with at least 10% Curse active.
Skull O'Maniac can also be picked up as an arena item within Mad Forest (located on the top-left, might make sense to get the Milky Way Map before trying to evolve this weapon to make it easier to find it).
If you're interested in seeing what the combo is for the other weapons they are listed below in the spoiler tag :)
*** Spoiler - click to reveal ***
Holy Wand – Magic Wand + Empty Tomb
Hellfire – Fire Wand + Spinach
Death Spiral – Axe + Candelabrador
Bloody Tear – Whip + Hollow Heart
Thousand Edge – Knife + Bracer
Heaven Sword – Cross + Clover
Unholy Vespers – King BIble + Spellbinder
Soul Eater – Garlic + Pummarola
La Borra – Santa Water+ Attractorb
NO FUTURE– Runetracer + Armor
Thunder Loop – Lightning Ring + Duplicator
Gorgeous Moon – Pentagram + Crown
Vicious Hunger – Gatti Amari + Stone Mask
Valkyrie Turner – Shadow Pinion + Wings