Melissa's Voice Studio

Piano Accompaniment Tracks:

https://pianotrax.com/

(in various keys, with piano-only and vocal melody options)

Sheet music for purchase:

http://www.mosfullertonmusic.com/

http://www.musicnotes.com/

http://www.onlinesheetmusic.com/

https://www.sheetmusicdirect.com/en-US/

Free classical music in the public domain:

http://www.imslp.org/

http://artsongcentral.com/

Music Theory Books:

http://www.musictheoryforsingers.com/index.html

Translations and IPA:

https://www.ipasource.com/