It's 'fiddly' but not impossible.
Connect the headphones output from a cassette player to the line-in input on a computer. (If there's only a microphone input use that instead but keep the volume level of the cassette player really low).
Record the interviews using Audacity, saving them as MP3 files:
http://www.audacityteam.org/
(NB: In its native state you can't save audio files into MP3 format using Audacity due to licensing issues. You have to install the LAME encoder, which will enable you to save MP3 files via the Export command in the File menu).
Youtube doesn't allow the uploading of audio files, per se. You have to make and upload a video file. The simplest way to do that used to be using Windows Live Movie Maker but Microsoft no longer offer it on their website. (I mention it in case you happen to already have it on your computer). This is the nearest equivalent freebie:
http://www.nchsoftware.com/videopad/
So then you need to include just a single image (of, say, Bowie or perhaps the radio stations logo), and your audio track, to make up your movie. (You could, of course, be more creative and use lots of different images).
Finally upload your masterpiece to Youtube in the usual way.