My L510 had two jacks, but my L520 only has one. I want to use the microphone on my headset, even if I cannot use the headset speakers at the same time. How do I get my laptop to switch from the built-in microphone to the headset microphone? The built-in picks up too much surrounding noise.
You will not be able to get the laptop to switch from the built-in microphone to the headset microphone without either getting a USB soundcard, or using a splitter.
This is because two jacks, which used to be a standard on older PCs, served as output for audio and input for mic, where audio went out and voice went in via your mic. This has been changed now due to recent trends.
To address this, you can either buy a USB soundcard with these two inputs. This may also improve sound quality as the USB soundcards provide additional gain boost.
Another workaround is to purchase a splitter such as one below.