Built-in or External Microphone Not working on Inspiron 8600

I have an Inspiron 8600 with Windows XP on it. The built in or external microphone was not working (Windows Sound recorder doesn't show any activity). The Dell diagnostics test also failed at the Sound Recording test. After replacing two motherboards the same symptoms reappeared after some time. Then one Dell Support person thought it could be caused by the modem, and he was right. After replacing the internal fax/modem, sound recording is working again.