I downloaded a 197 Kb mp3 file using FF.
When I played it on my Samsung S5, it only played about half the length of the file?
It played fine on my desktop.
Anyone know why ?
If it plays fine on your desktop, it means the file is ok. So either there’s been an error when transfering it to your phone and it’s been truncated, or for some reason, your phone can only play sounds up to a certain length. It would be weird, though.
If you have a file manager on your phone, check that the file is still 197 Kb. If you don’t have one, you can install one. I personnally use Amaze (it’s free software), which is available either from Google Play or from F-Droid.
File size is the same.
Just one of those mysteries. 
Is it shortened when you play it in a specific way (like a ringtone or an alarm)? Do you have a sound/media player on your phone? if so, is it shortened with it also?
It’s shortened when played by default media player.
I have no other media player.
I don’t know, then. It’s probably an Android issue. Is it the same with other sound files you try to play (music…)?
Have you tried transfering it again from your desktop to your phone (preferably in USB rather than Bluetooth)?
Found the solution.
I installed VLC.
It said my file was “wrapped with mp3wrap.”
I had merged two mp3s using mp3wrap.
So I opened it up with VLC and converted it to a “normal” mp3.
Now it plays on my phone.