[julie_m]

I'm assuming it's a stereo tape deck with a 5-pin DIN socket. Looking at your audio interface in another browser tab, I see it has two RCA sockets for input. Now, if you have a cable with four RCA plugs to a 5-pin DIN plug, two of the plugs will be the left and right inputs, and two will be the left and right outputs. You will be able to work out which output is which by playing a tape with different programmes on tracks 1 and 3, or just very good stereo separation. And you can ignore the inputs anyway -- at least until you are finished with listening to your tapes, and want to record something else over them .....

If you have a tape with different mono programmes on tracks 1 and 3, then you can record them together as a single stereo track and then split it into two mono tracks (if your recording software does not let you do this, Audacity does, and it is free and Open Source; I can talk you through the steps if needs be).

If the recorder is mono, it may well only be putting out a signal on one of the plugs. In this case, then you will need to get a coupler or "gender bender" for joining together 2 RCA plugs, a Y-adaptor with two sockets and one plug, and a pair of RCA to RCA leads. Use the coupler to connect the plug with the signal coming out of it to the Y-adaptor, and so split the signal between the two inputs which you will need to connect with the RCA leads. That way, you can record a mono signal as though it were stereo.

If the recorder is multi-speed and your tapes were recorded at low speed, then you could play them back at high speed, taking half the time, and slow them back down to normal in software taking less than that. You will also be halving the maximum recordable frequency, but in practice there will be little or no content above about 10 kHz, which would give 20 kHz when played at double speed -- still within half the sampling rate (22050 Hz with 44k1 for CD, 24000 Hz with 48k). Listen to the results; and if you are not entirely happy, redo at normal speed.

Try a known good source into the audio interface, if possible, and make sure you really can record a signal on your computer. The volume control may affect the signal going into the computer, not just the headphones. Also, it may have an additional, software input gain control in your settings somewhere -- see your documentation for details.