First of all, I like it!
Some, hopefully constructive criticisms, mostly nitpicking/paper cut bugs. My pedantism doesn't mean I don't like it ;)
- As somebody else mentioned, the email field on the signup page is a bit off-putting just to try it. Maybe add some text on why you need to know it (password reminders, whathaveyou). "Display Name" was already mentioned too for something that could be improved.
- I think it would be good to have a password confirmation box on signup. I have enough family that constantly mistypes their passwords to know that this is a thing :P
- When I created a box and was lead to the upload page (Step 2 of 4), I somehow assumed it would let me drag and drop straight to the page. Instead I had to click the button. Not a major thing but having a shortcut (like popping up that file picker once a dragover event happens) would be nice I think. Same for the Tracks page.
- After creating a box and uploading a track on the next page, it might be a good idea to pre-select that track ;) The list of selectable tracks is also a little too subtle. I had to think a few seconds what's going on; ideally I'd have to think less about it.
- It doesn't allow me to upload ogg or opus or straight wav. Again, no biggie, but the error text there could be more helpful. "The accepted file types are audio/mpeg,audio/mp3" is not so great. I later saw it says that "starters" are only allowed mp3 on the account page. Does this mean other formats are allowed when upgrading? I didn't see any info about that.
- The text listing the benefits of the different tiers on /upgrade is extremely thin and hard to read when viewed on a regular non-retina display on a Windows box. It also is different to the different tiers text straight on the homepage. And /upgrade has no favicon (tries to get /favicon.ico instead of /favicon/favicon-32x32.png that other pages use). It also shows me a $ price and an £ approximation. I'm on € and used to $, but seeing £ is mildly confusing. It also has a title of "Become a member", but I am already a member :P
- It now tells me: "You have exceeded the number of available SongBoxes on the Free tier. Consider upgrading." But the "Your SongBoxes" says "My Private Songboxes (0 of 5 in use)". I did create a few boxes and subsequently deleted them. Now I got none and am not allowed ot make any more. Also, shouldn't that be "(0 of 1 in use)" on starter? Bug?
- Speaking of which, you sometimes use "SongBox(es)" and sometimes "Songbox(es)", come to think about it.
- I like your privacy policy, short and simple. But it doesn't state if and for how long you will retain data after the user removes it. That would be a clarification worth making.
- Since you're pretty much about privacy too according to your privacy policy, a way to delete the account would be nice.
Would I use it? Maybe, but I am not a "pro" and the pricing is too expensive for the me (I am a cheapskate) at 10 bucks/month / 100 bucks/year for just 15 tracks. My price range for something like that, being "Hobby", would be maybe around 20-25 bucks / year for 50 tracks, but that's just me.