1.I wrote on my journal while I listened to (softly, soft) music.

2.Classes at SEO go by so (slow, slowly) during the summer.

3.As she presented her work, her voice sounded (different, differently).

2 answers

For number one it would be soft. ("on my journal" should be "in my journal" instead)
number two would be slowly
number three would be different
"would be"? Or "is"?