Exactly what is INSIDE the video you are editing?
Codec & Format information, with 2 links inside for you to read http://forums.adobe.com/thread/1270588
Report back with the codec details of your file, use the programs below... A screen shot works well to SHOW people what you are doing
https://forums.adobe.com/thread/1070933 for screen shot instructions
Free programs to get file information for PC/Mac http://mediaarea.net/en/MediaInfo/Download
Does your video use a VARIABLE Frame Rate or a Constant Frame Rate?
If your video uses a variable frame rate, read below for one solution, for iphone
Some people find that Iphone won't edit easily because it uses a variable frame rate
- A possible fix in message #22 http://forums.adobe.com/thread/934466
You do not need purchased programs to deal with any variable frame rate in your source videos.
On a mini test run basis, look at each of your sources in Premiere Elements after HandBrake conversion to H.264.mp4 with a constant frame rate (instead of a variable frame rate).
If you need how to instructions, let me know, and I will post them in the morning after you post information on what you have now in the way of video frame size and frame rate. Your webcam and cell phone video would be the most likely to be affected by a variable frame rate. Does the YouTube video alone present in the Premiere Elements project with audio out of sync?
What we need to put together also is the information on each of your videos properties and the project preset you are using for your Premiere Elements project.
(See Edit Menu/Project Settings/General.)