RETAIL — For 21 consecutive months, Sirius has maintained an edge over rival XM with regards to retail sales, says a new study by NPD Group. The firm says Sirius held 55 percent of the satellite radio retail share during May, while XM netted 45 percent. The study said Sirius sales were down 22 percent year-over-year, while XM’s numbers sagged by 26 percent. Overall, the entire retail satellite radio sector is down 24 percent from last year, NPD Group said.

RADIO — A coalition of internet radio sites will participate in a “National Day of Silence” tomorrow as a way to mobilize supporters to urge Congress to pass the Internet Radio Equality Act. The legislation looks to correct an earlier ruling by the Copyright Royalty Board that will increase rates for internet radio so much that most would be forced to shut down. Yahoo!, MTV Online, Live365, Rhapsody and Accuradio are among the thousands of sites scheduled to go silent.

REGULATORY — The Federal Communications Commission’s Enforcement Bureau terminated its investigation into Telenor Satellite and whether the company violated certain sections of the Communications Act of 1934. The agency found there were no substantial problems with the matter and that the public interest would best be served by terminating the investigation.

RESEARCH — A new report from The NPD Group says Boston, New York, and Orlando are the top markets for GPS devices during the first quarter of 2007. According to the study, retailers in those cities sold more mobile navigation unites on a per store basis than those in any other of the country’s leading DMAs