Microphone will record but not play over speakers

I've been experimenting with transferring some material I have on cassette tape onto my Aspire using a stereo cable from a tape player to the microphone input. Audacity will record it with a bit of tweaking of audio levels - it's in stereo and reasonably good quality.

But on my old computer (a Dell Inspiron with Windows XP), I could hear whatever sound was being inputted on the microphone over my computer speakers, and I can't do this at the moment. I've checked the Volume Control settings, and made sure that "Capture" isn't Muted, but still nothing. Any ideas?

Thanks!