Re: low audio problem

The hissing is just the background noise floor. Could actually be caused by many things, including inherent electronic noise (my guess as to what it is). You won't be able to eliminate it, but you can mitigate it somewhat.

I'd attack it with a simple equalizer first: "hiss" implies it's high frequency and broad spectrum. Knowing that a the human speaking voice is anywhere from 80-250 Hz, you can use the EQ to lower higher frequencies. Many EQ plug-ins (including Vegas' Track EQ and others) will have presets, like "Hiss Reduction" or something. Make sure you toggle the filter on and off while working to hear how badly you are affecting the voice as well.

You can also go after a problem like this with a noise gate, which will quiet everything below a certain level. This is much trickier and can be obtrusive, with the noise floor turning on and off as the person speaks. Might be what you need however and can work very well, especially if you throw a music bed underneath.

Which brings me to my final bit of advice: throw a music bed underneath to hide the hiss, if it would be appropriate for your video.

And a final bit of un-advice: there are fancy expensive noise reduction plugins. They can work, but they are definitely not magical (despite the marketing claims) and are most effective when used in combination with EQ and maybe a soft gate anyhow.