heres what i did today with same issue, also affected my alarm clock ringer.
my situation was this am i 'overslept' right through my alarm (os 5 - set to alarm antelope) or so i thought when i woke up late. but then i saw that my bb screen showed did i want to dismiss the alarm or snooze it? checked options for alarm settings in clock, was set the way i expected: my setting of alarm antelope was unchanged, volume and duration same. clicked to test it at bottom and got popup that this ringer is unavailable for the phone choose another. went through several ringers - they all came installed on bb, and all were 'unavailable'.
Immediately decided i need to know that the phone ringer is working, and found exact isssue with my phone ringers.
so did a battery pull, left it out a good 5 minutes, rebooted and all was normal. i didn't think of playing ringtone with media player, not sure what diff it would make though?
am positive phone ringers worked last pm several hrs before i went to bed and no missed msgs since. no new apps on phone to blame from time ringers last worked. so...
it's working again, but i'm ver concerned cause no clue what caused it and need to be able to rely on bb. is this a 9650 'new' issue? after a yr and a half, what gives?