On your MacBook, under System Preferences, Security & Privacy, General, check the settings under: Allow apps downloaded from...
On my Macs it is set to "Mac app store and identified developers"
Updating the OS to El Capitan shouldn't have changed your previous setting but one can't be sure what may have happened in the interim.
Consider buying/using the AIFF file format for your downloads. Yes, the files are larger but require no conversion to be compatible with Mac (iTunes, right?) The cost of hard disk space has gotten relatively low. Now I buy and/or rip to AIFF in order to have maximum compatibility with present and future players.
On my Macs it is set to "Mac app store and identified developers"
Updating the OS to El Capitan shouldn't have changed your previous setting but one can't be sure what may have happened in the interim.
Consider buying/using the AIFF file format for your downloads. Yes, the files are larger but require no conversion to be compatible with Mac (iTunes, right?) The cost of hard disk space has gotten relatively low. Now I buy and/or rip to AIFF in order to have maximum compatibility with present and future players.