I have an older non HD TV with A/V jacks and want to use it as a monitor with sound. I want to use a USB to A/V cable to do this. Will that work?
USB is digital and A/V jacks are analog, so I don't believe you will be able to find an A/V cable to do this. Also, USB is not a video standard, so you would need to find a USB -> AV converter. While these devices probably exist, you need to be a bit careful as most of the things with USB and A/V jacks will be to capture analog TV, rather then output it.
Despite substantial Googling, I was unable to find a USB device which would do what you want. The closest I could find was a VGA -> RCA converter - which is more expensive then a replacement video card !!! (I'm not recommending this product, just linking to it for your ref - http://www.amazon.com/StarTech-com-Resolution-Composite-Converter-VGA2VID/dp/B001NXDQEK)