My wife's Nexus 5 recently started behaving very weirdly.
For notifications, it always think that it's in silent mode (i.e. it vibrates and doesn't play a sound). This happens regardless of the actual volume settings. All other sounds seem to work properly (ringtones, in-call volume, etc.)
Also, when she presses volume up or down, it adjusts the in-call volume instead ringtone volume.
I have tried rebooting, and I even tried deleting the cache (as described here - https://android.stackexchange.com/a/139739/165076) but nothing seems to help.