Looking for a Music mixing App!

I'm not an expert on the subject, but I've been using Magix MusicMaker 15 since 2011 as a music writing tool. Since I have no actual recording equipment I've been using some virtual instrument that came with the program and other free ones I've found online. It includes a few preexisting loops and I'm pretty sure you can also write using a keyboard via USB (although you can directly write music into it even without one). I don't remember the price I paid though and if I understand the terms of service properly, you can't use the premade beats or virtual instruments that come with the program for non-personnal uses.

Due to that last part, I've been recently using LMMS to create the music for the few speedpaint videos I've made. I would recommend you to try it out since it's free to download and it works in a similar way than MusicMaker. Although I find it a bit more tedious to use (and it did crash a few times), I feel like it contains some interesting functions and instruments that I don't think exists in MusicMaker.

And I'm sure I saw at some point that there are better ones out there, but I unfortunately don't remember their names. I hope this will help you anyway.

I searched a bit and found this part from the user's license (keep in mind that my version of the app is old, so it might have changed):

6-Use of music, video and photo files:
The music, video and photo files included with MAGIX products may only be used within the scope of producing personally created works to be used for non-commercial purposes. This also applies to music, video or photo data acquired through or by means of MAGIX products. Commercial use constitutes receipt of financial benefits through direct or indirect use of the files (through sale, licensing, ad banners, etc.)

And if I add this article I've found http://magazine.magix.com/en/commercial-use-of-music/ it seems to imply (to me anyway, I'm really not a lawyer) that using it to create music for youtube videos or other websites is fine as long as the video is not monetized or you don't get any revenue from the websites where you posted it.

Yep, you are right.

Ableton Live is one of the best in my opinion if you want to record sounds and create songs (more than just mixing). The "bad thing" is that it's not free.

I think you're probably looking for a free app, so this probably doesn't help you a lot, but is what I can share about that ^^ :

I used Ableton long time ago to make music and some BSO for shortfilms using beats from internet and making my own sounds, recording piano, guitar and other sounds (some crazy folleys I was asked to do, don't ask please xD). Also I tried so many softwares as Pro Tools or Reason in different jobs but for what you're talking about I found Ableton the best one.

Now I don't play the any instruments, I don't create songs neither make any strange folley, visual arts eat all my time xD But some of my friends that are working in the industry are using it so I think the softs still similar than before.

All depends in what you're looking for, same as 3D softs , if you want just to try and play the piano , probably they're some free softs better for it. Audacity I think is one of the most famous but I don't know if it works well for mixing but I'm sure there are some easier apps as the one cedricgo says to just play the piano while playing some beats.
You can try Ableton for free 30 days, and they have an 80€ licence, but look well if you're going for it couse probably you can't do something you want with this short licence.

Hope it helps (I think not a lot) at least a little "beat"... omg, ok, I stop here. ^^
Let us know what you finally did!