How to fix unknown error (-101) and sound

I use dropbox frequently and recently tried to play a video I uploaded and was not able to.

So I emailed it my self to upload it to my MacBook Air to edit it in IMovie and I received the following message:

I downloaded a quicktime update and tried to restart my computer a few times... no luck.

Inaddition I have lost my sound.

Can anyone help?

Thank you