First of all – there are more than one controls that can affect volume. 

1. There is a volume control right on the video – at the bottom of the box that contains the video as it plays.  It usually looks like a round icon of a speaker ..l. hover over that and see if it is set all the way at the top.  
2. There is a volume control on your computer screen at the bottom right – it looks like a speaker in profile, ‘the mixer’ – click on that and be sure it’s all the way up.
3. Some computers have hardware controls for volume.  For example, Toshibas usually have a small wheel that you can spin one way and the other to change volume up and down.
4. If you use a headset (a good idea if your computer can’t give you enough volume) there may be volume controls on the headset too.

If none of that helps, use the ‘Contact us’ page to tell me the names of the videos that are too soft and I’ll check them out from my end.