Cassette tape is such a shabby medium that you might want to do some cleaning up, like noise suppresion and a little eq if needed, normalization etc. You can do that in post production (i.e. after recording it on HDD but before commiting it to CD). Since it is voice, specs need not be that high. A bitrate of 128 or 160Kbs will do just fine. There are freeware apps that can do this for you.