I own a set of Jabra Revo Wireless headphones, there great, but the manual for it is less so.
Tonight, I somehow triggered a setting which turned off all sound prompts, helpfully the headphones said 'sound prompts off', unhelpfully the manual had no reference to this at all!
After a lot of faff, I worked it out. Hopefully this helps someone:
Jabra Revo Wireless sound prompts can be turned on by switching the device off, pressing and holding the multifunction switch (right side button), then switching the device on. The device will say 'sound prompts on' and its business as usual. (Switching the prompts off again is a repeat of the above)
Jabra, you could do with adding this to your manual!