C++ for dummies book
Those dummy books typically dont cover everything, I am not sure if there is even a up to date version for C++, im not saying they are bad, they imo are excellent basic books and have a really decent way of explaining to beginners but you might want to consider a more advance book after reading it and practicing a lot before you consider making a game / audio editor.
First of all I want to make a 2D RPG/SideScrolling game, what API do you suggest?
This section is flooded with questions much like this, you will likely get different answers for different reasons, most of the reasons have been mentioned in other threads and theres usually about 3 "what shall I pick?" threads or similar created a day.
directx, opengl, sdl, sfml, ogre, panda3d etc. They all have their pros and cons, theres obviously other options, some that require minimum or no programming.
Im a DirectX fan but I think its overkill for what you require
Also i want to make a audioEditing and music creation software, what API would be suggested for that?
This could be achieved with Fmod however im not certain, I havent done much work with audio programming but if I were I would probably look to see what fmod can offer
As for making apps, while this may not be the best option based on your language choice, I would recommend WPF, never used Qt but I guess thats good too.