Audio cassettes use the same basic technology as VHS tapes — a thin layer of magnetic oxide particles bonded to a plastic tape base. And they suffer the same fundamental problem: the binder that holds those particles in place breaks down over decades, causing the oxide to shed and the magnetic signal to weaken.
But audio degradation is insidious because it's invisible. You can't see a cassette losing its signal. You can only hear it — increasing tape hiss, dropout in quiet passages, loss of high-frequency detail, and a gradual muddying of the overall sound. By the time degradation is audible, significant signal has already been permanently lost.
Cassette mechanics compound the problem. The pressure pad that holds the tape against the playback head deteriorates, causing uneven contact and inconsistent audio quality. Internal tape guides warp. The slip sheets that reduce friction between the tape pack and the cassette shell dry out and crumble. All of these mechanical failures affect playback quality even when the tape itself is still viable.
For cassettes containing irreplaceable recordings — family voices, personal dictations, one-of-a-kind live recordings — the window for high-quality transfer narrows with every year of delay.