I would like to know about the legal side of using guitar solo cuts inside a mobile app. The solos will be cut from commercial music of rock bands. The app will be free.
Can I use solos like that?
No you can not do that. It is a violation of the musician's copyright on their music.
Also note you can not record that same music yourself and use that in your app- without paying royalties to the people who wrote the music.
To use someone else's property, you have to pay for the right to do so. And they may not allow you to use their property.