If she and Reindeer Kog'Maw are close to each other and both play their jokes four times after another with Sona going first, they will play and sing an excerpt from Deck the Halls. A video demonstrating this can be viewed here.
She has three toggleable ('/toggle' or Ctrl + 5 by default, second row from the bottom in Options > Hotkeys > Communication > Toggle to rebind) forms (Kinetic, Concussive, Ethereal)
Changing forms a 0.5 seconds cast time and a 2 seconds global cooldown (this also triggers while in combat)
Each one has a 'theme' that is played in layers as the game progresses.
The layers played will be added to all songs depending on what basic ability is used for the first time and also which level of ultimate has been used for the first time, the latter also affecting the theme structure.
Sona's teammates can opt in to listen her soundtrack from the scoreboard (icon to the left of her portrait)
They cannot choose which 'theme' since that is dependent of the form Sona chooses.
Using Crescendo for the first time as soon as an ultimate level is unlocked will upgrade the theme structure of all forms. Using the level 1 ultimate will permanently apply all basic layers, allowing the basic abilities to add new ones. Using the level 2 ultimate will allow the song reach its climax, becoming the regular version of the songs. Using the level 3 ultimate will trigger a more dynamic and intense version of the regular songs.
If Crescendo is used for the first time while in Kinetic form, a fragment of the Metroid theme will play.