I LOVE this suggestion by David G Stone - He suggests that you get all of your students to record on their iPads/Chromebooks. (Screencastify and Flipfrid seem popular on Chrome.) All at the same time, the students hit record and introduce themselves. He then plays an intro and has the entire class sing a bit of a song. He says you can hear everyone, but mostly the one recording. This could be modified to cover all sorts of singing assessments!
Quizizz - I use this a lot for formative assessments. The kids can play and compete in class (This is a lot like Kahoot, but I personally like it better.). You can go back later and download a spreadsheet of their scores and enter them as a grade. This is a great way to have a fun moment while quickly assessing where kids are at. I'm building a new set of quizzes, but here's a small collection of mine you can use!