Thank you I will do it, what website can I use to reach them?
This is fine for future reference but will not resolve the OP's current problem.
I believe what HarshVardhanSolanki is suggesting is to store your file resources online, and link to them as required in your app, this would then mean they are not in your aia project.
It may be better to read this:
https://ai2.appinventor.mit.edu/reference/other/usingImages.html
Then once you are ready to upload assets to your project, resize all your media assets accordingly Including any sound files) to suit the requirements of your app. The ai2 server will allow for an aia project size of @27mb.
If I am going to link my resources from online, that would mean my app will not work offline anymore?
yes you will need internet connection but you can add a download button to store selected songs that the user would like not the entire collection
How many and what is the size of these MP3s?
The limit for an AIA is 30MB (and 50MB on http://code.appinventor.mit.edu/). So remove MP3s if necessary.
Two options to avoid that your app will not work offline:
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Download MP3s to the ASD or internal storage at the first run and access them from there. (For this internet is only required for the first run of the app).
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Remove almost all MP3s from the assets and follow this guide: How to build a big app → APK max. 100 MB or → AAB 150 MB