Another thing, sometimes crippled demo versions of plugins will shoot occasional white noise into the signal path as a way to encourage you to buy the full version. Are you using any demo versions of plugins?
I would go further to recommend a cheap usb microphone. Even the cheapest of the cheap would most likely do better for you than going direct into the laptops aux input.
I watched a video where the ME deliberately did his metering/analyzing/etc. with a different computer set to the side so that he would deliberately have to look away to see what was going on. Claimed that it kept him from mastering with eyes only.
Mastering while you mix is a very bad idea anyway. They are two separate stages of production and should be treated as such. It's even better to have the luxury of having mixing and mastering performed by different engineers. I do master my own stuff (crude mastering also) but I have to step...
I believe a better software solution would be using Reaper with ASIO4ALL. Both are free downloads. (Reaper will nag you for a few seconds at startup until you register it but it is cheap if you decide to register.
Google ASIO4ALL and download the driver and install it. Then chase down Reaper and...
I have been here for years, haven't learned anything and have no friends. Being that the rope broke this evening as I tried to hang myself, I will take a minute to say hello.
Or just get a PC and an interface. I don't understand this shunning of computer based DAWs that we occasionally see here. I'm not saying it's the future, because I first have to say it's the PRESENT. :)
Another vote for Reaper. I don't use it myself but only because I got used to cubase first. Also, another vote for studio monitors and I too have KRK's. They get the job done for sure. Many will agree, mixing on headphones is a bad idea.
Start light until you get your feet wet and decide...
...Join my new FB group here so you can stay tuned to all the flame wars and arguing about analogue vs digital and occasionally learn something all from the convenience of your favorite self-whoring....ahem....social networking site.