One technique used by many a DIY dabbler back in the glory days of Cassette Culture was the blind overdub. This involved covering the erase head with a piece of paper or card. This allowed new layers to be added on top of the original recording until thick walls of sound were created. Each new recording partially obscured the old until, if enough blind dubs were done the initial track would be totally obliterated. However I do recall that curiously if one carried on long enough sometimes ghost-like the original track would re-emerge from the soup.
An overdub delight entitled "Nein Nein Nein" recorded by Steven Ball and myself back in 1980 used a similar technique but with a 5 minute answering machine cassette. These tapes are cassette loop and so one gets looped blind dubs. The output of the loop was then fed into a ring modulator. "Nein Nein Nein" can be heard below.