Check your System Preferences->Sound settings. They may be set to "0" - or no output device. You could also try tossing ~/Library/Preferences:com.apple.systemsound.plist. When "things" (apps & settings) behave strangely it can sometimes be due to a corrupt preference (.plist) file.
As a workaround use a USB dongle. USB audio is separate from the analogue audio. These dongles typically have a microphone in addition to line in/out ports.
You can also increase the number of USB ports with a self-powered USB hub. You want a self-powered hub because simply adding a hub will draw too much power and may damage the USB port(s) you have.
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