how can you tell battery is run down

I have a 2007 MacBook and when I play videos they sometimes start sounding garbled after awhile. Though not right away. They sometimes look weird too, like they are dissolving, but not always. The sound issue is the more constant one.

I have also noticed that my computer is needing to be shut down more as it is freezing, web pages are freezing. Or, it is too slow. I am using Chrome and FireFox. Mostly Chrome and now and again Safari. All have this problem. Is it a battery issue or?