I've not found it written down anywhere, so wonder if anyone could tell me the right way - have got it to work by unlocking vid and snd tracks, but everytime it's a bit hit and miss.
What i do is select the event i want to edit, press U to ungroup the audio & video, make the necessary edit, then select both the audio & video and press G to group them back together again. That way you don't have to worry about accidentally moving anything else around.
DON'T FORGET TO PUT BACK - "Ignore Event Grouping" - afterwards . . otherwise you will automatically age 28 years. I usedto be . . . I'm now 104! - Believe me!