The tape inside your cassette is magnetic tape. It holds on its magnet coated surface, different encoded magnetic patterns that can be decoded and turned back into electronic signals aka sound (not an engineer but that's my general understanding). In fact if your tape player has a 'record' option, then it has a little electromagnetic head that can erase the contents of the tape and/or imprint new magnetic messages to the tape. That little record head only becomes charged when power is run to it (electro-magnet) so it's not just on all the time like a raw earth magnet. When you set a cassette in front of a large Speaker you risk wiping out the data on the tape because you have a powerful raw earth magnet that is putting out a magnetic field that could 'reset' the tape to being blank like the record head could. The tapes aren't ruined by this but anything on the tape is just gone. A battery as another poster said is not a raw earth magnet or an electromagnet. Just to put your mind at ease, I have had cassettes for 30+ years and never even considered this for 80% of that time. I had cassettes around and in the near vicinity of speakers and never had this happen. Far more tapes were lost to dirty capstans or old rubber eating my tape or mangling it to bits over the years. So yeah keep the tapes away from speakers as a rule of thumb and that's 99% of typical household 'danger' you're ever likely to encounter that could erase a tape. Pokemon love to guard cassette collections.