QR Code can contain a maximum of 4096 characters
Theoretically, you could put an MP3 into a static QR Code if the file is less than 4096 characters. That would be just a little more than a second in poor quality.
QR Code readers are not audio players.
Even if you encode an MP3 into a static QR Code, it would not be recognized by a conventional QR Code reader and would only be displayed as text. QR Code readers are not audio players.
Alternatively, develop your own app that has MP3 files stored offline
If you have a QR Code to play an MP3 offline, you need an audio player app that decodes the QR Code. It would be better if the app itself saved all the MP3 files and the QR Code only contains the title that should be played and tells the app this by scanning the QR Code from the app.
That said the best way to play an MP3 after scanning a QR Code is to link to a website where the MP3 file is stored.