There might be many common test cases: Here are 2
I)
1. For SMS: It uses UDP protocol
2. for MP3: Audio streaming also uses UDP protocol

II)
1. for SMS: For entering text for SMS, you need to open 
Text editor.
2. for MP3: For entering/editing play list names, text 
editor should be opened.
So, the test cases written for text editor can be used for 
both applications.

1.User should able open the SMS or the MP3 application.

2.device should behave properly for all cross trigger scenario.
For example if the SMS or MP3 application is opened 
   a)a Incoming Call arrives
   b)A Alarm trigered
   c)A SMS/MMS arrived.

If its MMS,

Please attach a MP3 audio file to the MMS and sent it.At the recipient end play the MMS Audio file.

You have send/receive of MMS and also playing of MP3 file.