I had this problem with my Big Bang Xpower II by MSI. The onboard sound chip was so quiet it drove me nuts.
The cheap way -
https://www.overclockers.co.uk/asus...rd-with-built-in-headphone-amp-sc-015-as.html
Probably about as good as onboard, only it has a headphone amp built in which is pretty good. I have a higher end model (forget which one now) and even for a basshead/sensory overloader* it was sufficient.
The slightly more expensive option.
https://www.overclockers.co.uk/crea...ound-card-retail-70sb150000001-sc-087-cl.html
The Z can be found on Ebay for £40. I know the purists will argue that it's crap, but it's actually one of the best sound cards I've ever used. I say that having owned a Asus D2X at launch when they were £300+.
I prefer the Z's software and drivers tbh. Whilst I've not had any mad issues with the Asus 7.1 card I have it did stop working once and nothing I could do would fix it. Only by removing it and putting it in another PC did I get it working again.
But yeah, the Z is a great sound card.
OK moving onto the more expensive option.
http://www.amazon.co.uk/gp/product/...ne tube dac&qid=1453041477&ref_=sr_1_1&sr=8-1
An actual stand alone DAC/Headphone amp. I use that one ^ because I like the sound of tubes/valves but there are many others, some cheaper. The purists will argue that you absolutely must use a DAC if you value audio quality. The Aune runs in USB mode completely replacing your sound card but I had issues with it in a couple of games. The easy fix was to simply run it in 'line' mode, reverting back to my SB Recon as the source.
* I like my music so loud that if some one screamed at me I would not hear them. I like to literally overload my senses so that I can not think nor hear anything else other than music. So the Asus and the Aune get my thumbs up, as with both you will probably quit before they do.