[ben]

If your memorex tapes are the early grey ones with the L-shaped box, then the pressure pad sponge has likely deteriorated and won't push the tape up against the head fully. Some other memorex tapes like the 'HBS' ones also use this damn pad! You can try replacing it by gluing a small piece of felt in there. Fiddly but do-able-

Later tapes used a small piece of felt on a springy brass strip which are better, though the pad can sometimes fall off these giving the same muffly audio.

Some decks handle these better than others, if they have a small amount of 'backtension' (drag) on the supply spool then this can keep the tape taut across the head.

We discussed this on a recent thread, search for it if you are curious!