Could CDs be replaced now by a fingernail-sized memory card?

Perhaps not entirely, but SanDisk Corp., four major record labels and retailers Best Buy Co. and Wal-Mart Stores Inc. are hoping that albums sold on microSD memory cards will at least provide an additional stream of sales. The companies were expected to unveil plans Monday to sell memory cards loaded with music in the MP3 format, free of copy protections.

Called “slotMusic,” the new format is meant to address two intertwined trends. Most albums are still sold in a physical format — 449 million were sold on CDs in 2007, while 50 million were sold digitally, according to Nielsen SoundScan — yet CDs are decreasingly popular. Albums sold on CD dropped almost 19% last year.

Given this, the record labels — Vivendi SA’s Universal Music Group, Sony BMG Music Entertainment, Warner Music Group Corp., and EMI Group PLC — are hoping slotMusic can be another physical revenue source — and one that is more versatile than CDs, given the kinds of gadgets people carry around these days.

Unlike when the CD was introduced and people had to buy new players, many people already have the ability to play slotMusic albums, since many cell phones and multimedia players support microSD cards.

These new albums will come with a small USB dongle that lets buyers use them with computers, too.

Albums will be sold on 1-gigabyte microSD cards, which means they will be able to hold a full album and related content such as liner notes and cover art. Buyers will be able to use extra space on the cards to hold songs and photos from their own collections.

It’s not yet known exactly when — or how many — albums will be initially sold in the format. The albums will debut at Best Buy and Wal-Mart stores in America, and later in Europe.