For copyright, a program and the output from a program are completely independent works. The copyright license under which you receive a program has no bearing at all on the licenses you can use for the work that you create by using the program.
For example, the GCC compiler is licensed under the GPL license, but there is no problem in using it to create closed-source commercial applications. Also, MS Word is licensed under a closed-source license, but there is nothing stopping you to write an open-source novel with it.
The only exception here is if the output of the program contains not just (a mechanical transformation of) your input but also copies of parts of the program itself. The most well known examples of such programs are parser generators like Bison.
When you create an MP3 file, the ownership and copyright status depends on how you created the file.
- If you start out with one (or more) samples from someone else, which you then modify to create your MP3 file, then your file is a derived work of the source samples. In this case, the copyright of the final MP3 lies with both you and the creators of the original samples and the license that you can use to distribute your MP3 under depends on the license under which you received the original samples.
- If you create the MP3 file from scratch, by recording your music or creating it yourself, then the copyright is entirely yours and you are completely free in your choice of licenses.
When publishing an app that also includes music/artwork and either of them falls under an open-source license, then it is recommended that you use separate licenses for the app itself and the artwork and/or music. The reason for this is that all open source licenses are either written for use with software (and they generally use terms that don't have a well-defined meaning outside the context of software), or they are written for use with artistic works. By using separate licenses, you don't get into the situation that the license contains terms that are not really suitable for half the project.