Here is a collection of online audio/visual tools that I've used.
They tend to disappear over time so if one isn't working any more, drop me a note and I'll tell you if I've found a replacement.
Sometimes I want to "zoom in" a video before I share it. The way I do this is to crop the video just like I would do a still. Like a still, I lose some resolution but it gets the focus where I want it.
Video-Crop is what I use.
Like their web site says "You can find here damn simple and juicy tool for video cropping."
Remove Audio from Video
Especially in my dashcam video sometimes there's audio that isn't appropriate to share. AudioRemover is what I use.
MP3 from Video
When I come across a video, usually YouTube, that I want to extract the audio from and make an mp3, convert2mp3 is what I use.
If I have an mp3 that I want to trim down, e.g. for a ringtone, I use mp3cut.net.
To increase the volume of an audio clip, e.g. for a ringtone, I use mp3louder.com.
I can choose how many decibels I want to increase the volume.
YouTube to MP4
If I want to save a YouTube video, I need to convert it to a format that I can store. I use ClipConverter to do this.
Notice that ClipConverter has a variety of output formats.
Audio to MP3
I have some old audio files (wav, amr, etc.) that I want to convert to mp3s. Online-Convert does it for me.
Anything to Anything
When all else fails, I try CloudConvert.