Tuning Videos:

Bass:  https://youtu.be/mozx9sUdSZc

Correct Instrument Position: 

Violin/Viola: 
https://www.youtube.com/watch?v=UhlQY7rPTzk

Cello: 
https://www.youtube.com/watch?v=dIdotoGZ3F8

Sound tracks:

Essential Elements: Book 1 recordings #2-32
https://www.youtube.com/playlist?list=PLAg2lxz7fXEjMNfKCPFxEj1i-5Pp3l0gj

Web Resources: 

Check out my friend and collegue's Strings Resource page on YouTube!  His songs use mostly D string and A string notes!  He JUST added "Eye of the Tiger." 
Link:  https://www.youtube.com/channel/UC61GP4K0yjJnnXw3udPpgbA

https://virtulessons.com/play - Note reading game

 https://pbskids.org/games/music/ - 
PBS resource for music games for younger students

https://www.sightreadingfactory.com - (Offering free trials)  Exercises for sight reading and sight singing practice

http://www.musicracer.com -  game site to practice musical notes and terminology