I also have a Dell Inspiron. My model is 1545. Previous versions of Ubuntu 12.04 (last year) had sound input from my internal microphone without any action on my part. The current update to version of 12.04 no longer accepts the internal microphone unless I use the methods described below. Thus, I used Skype, and sound recorder successfully on previous versions of 12.04 and can use the internal microphone on Windows 7 without intervention, but have to intervene on the latest version. (My laptop is dual-boot). My version of Ubuntu is KXStudio.
I found the solution for this problem on the current 12.04 version by looking at the first entry in the questions section:
Internal microphone not working
If you do not have Pulse Audio Volume control, go to software center and download/install it. Once you have installed it, check your volume control to be sure the volume is OK. Note: The small microphone symbol for each microphone input is effective when it is highlighted in a grey darker than the surrounding window. Otherwise it is mute.