Digital-music sales in Japan appear to have plateaued.

In first-quarter sales data released today, the Recording Industry Association of Japan says its 48 member companies sold 118.3 million units of digital product in the January-March period, down 2% from the first quarter of 2008, for a value of 22.5 billion yen ($232.9 million), unchanged from last year.

The lackluster results were mainly due to a 19% drop in mobile-based master-ringtone sales to 41.3 million units, for a value of 4.5 billion yen ($46.1 million), down 24%. Sales of ringback tones, in contrast, rose 24% to 28.1 million units, for a value of 2.4 billion yen ($24.8 million), a 28% rise over first-quarter 2008.

Sales of mobile-based full-track downloads increased just 1% to 34.7 million units, for a value of 11.8 billion yen ($122.7 million), up 4%.

Overall, mobile-based downloads fell 4% to 106.9 million units, for a value of 19.6 billion yen ($203.5 million), down 2%.

PC-based music downloads, although still dwarfed by the mobile sector, showed healthy if somewhat slackened growth, increasing 15% to 11.4 million units and 2.4 billion yen ($25.4 million), in value, up 18%.

PC downloads comprise singles, albums, mini-albums and other content sold over wired networks, according to the RIAJ, and albums and mini-albums count as single units regardless of the number of tracks they contain. Singles, however, constitute almost all PC-based download sales.