Firstly, you need to locate where the MP3s have been placed. To do this, install ES File Explorer File Manager and use the built-in search function. Type
*.mp3 and let it scan. Once the files are found, move them to a more accessible place. Afterwards, install Dropbox and create an account. Transfer the files to Dropbox, then switch to PC and load Dropbox website. Login with the same account and then you can download the files.