I used to use "bulk tape erasers" to do the job in less than a minute for standard cassette of any length, 8 track and even reel-to-reel magnetic recording tape. Otherwise, you'll need to record "silence" over the entire length of tape. That will be time consuming - but will make the next recording clearer as opposed to just recording the dictation over the old recording.Here's a
link to some I found on Google. I'm sure there are plenty of other places to check (like ebay or craigslist) too. Don't forget to demagnetize the recoding head too! There are tools for
this that will dramatically improve the sound captured on the tape.
Here are a few I found. Finally, physically scrubbing the head with a plain old Q-Tip dipped in denatured alcohol to loosen dirt and grime that collects on the head and anything that comes in contact with the tape itself. You can get that,
here. Doing this along with replacing old tapes (they get stretched out, scratched, and dirty) regularly will keep the audio sounding crisp and clear. Good luck!
