I have connected a tape recorder to the motherboard of the Mac OS X (10.4.11, Power PC G5), and I am trying to listen to a cassette tape through the external computer speakers.

So far I have not been successful, but there is visual evidence that the connection is getting through. More specifically, the input level in the sound window is visually active or flickers when I press play on the tape recorder, and the same activity on the input level stops when I stop the tape recorder.

Here is a summary of the audio troubleshooting I have done in trying to solve this problem:

1. Ensured the mute button is not check-marked within the input and output windows
2. Ensured the input and output volume slider levels are moved fully to the right
3. Noted that volume in the external speakers works for a variety of other sources, including online audio and video podcasts, online radio, and all iTunes audio sources.
4. Verified that there is audio on the cassette tape itself by playing it on stand alone cassette tape player.

Any additional troubleshooting tips are greatly appreciated.
Thanks in advance.