The ringtone constantly switches itself off so I miss calls and notifications all the time.
So annoying I have to keep checking the ringtone is on.
I have just missed an urgent call from my daughter because of this.
Have you tried a factory reset
Are you hitting the volume up/down when locking/unlocking screen?.... This seemd to be the most common reason when I checked the Android forums.
Also, are you using ringtones on the external sd card?
The device can sometimes lose track of these and will go silent.
Have a look at this thread...
Veritas Numquam Perit