I want to start uploading my Qur'an videos outside YouTube, is it halal to upload it on SoundCloud, where there are lots of music? Or on Vimeo and Instagram, where there are haram videos?
You are asking if it is okay to upload videos in sites where there are music videos, yet Youtube, which you upload (according to your saying) your videos on, is full of music and inappropriate videos, so what makes Youtube Halal and such sites Haram?!
Actually, and it wasn't that long when Egypt started making something that is called Educational Cafes (in Arabic: المقاهي الثقافية) where a number of Sheiks (leaders in a Muslim community or organization) would visit cafes and start interacting with people by discussing Islamic related issues, advising them, leading them to the right path (the real Islam), and the like.
Surprisingly (or maybe not), the first attempt of doing that was a huge success that Al-Azhar University (which was the founder of that activity) started doing the "Education Cafes" in many other regions in Egypt.
What is strange about that is that cafes in the past few years till now are known to include people (men) who are escaping life problems (such as financial and relational ones) and usually suffering several mental disorders. So, the kind of people there would be really depressed of life and could have a very weak relation with Allah (thus they ended up that way).
I'm stating facts in the preceded paragraph, and I'm certainly not trying to disgrace anyone. The source has been linked as a reference in what I've stated.
Here is another real-life example which is a Youtube video where a Sheikh attended a wedding party in which he wasn't invited to. It wasn't an Islamic wedding, so he took the microphone of the DJ with the permission of the groom, and after praising Allah Mighty and the prophet (Peace Be Upon Him), he started talking about the life of married couples in Islam, how it should be, and so on. He was very welcomed by the guests there from the start of his speech to the end. And when he wanted to go as to not be a burden on some people, they pleaded him to stay and prompt him to carry on his speech.
A non-Islamic wedding tends to have a very loud volume of music, partying, singing...etc which are all as far as what an Islamic aura would be. So, starting an Islamic speech and creating an Islamic atmosphere is indeed way too far from getting approved and accepted from the people there, which wasn't the case here.
Thus, if you wanted to spread Islamic videos (whether they were Qur'an recitations or about Islam teachings or anything else) in sites where Islam is barely mentioned, then you can do that, and who knows, you may guide and/or convert several people to Islam with God's permission (بإذن الله).