Amazing App- Smart Volume Control. Seriously awesome app... The amount of automation in addition to command over the amount can be wonderful. Swallows a little starting, due to the several choices it gives you yet once anything can be changed you never need to hint the amount all over again.
You just have to download this app and it's totally free.
Here's the link Smart Volume Control
The volume up/down buttons should certainly work. Have you installed any apps which are able to take control of those buttons? Games? Camera apps? One may have grabbed control and not relinquishing it - stuff happens, especially with a newly released operating system version.