Sonny Bono, who appeared along with Cher in "The Secret of Shark Island" (The New Scooby-Doo Movies), was a well-known pop singer who achieved international fame. A lesser-known fact, is that Sonny dropped his music career later in life to become a politician in the United States. Sonny was the mayor of Malibu Springs, California for four years from 1988-1992. Sonny later pursued a seat the U.S. Senate, though he ended up not getting the nomination. To this date, Sonny Bono is also the only U.S. Congressman to also have a number 1 single on the Billboard Hot 100.
You most likely are also aware that Sonny and Cher did not allow their episode to be released in the US on The Best of the New Scooby-Doo Movies DVD set. Intriguingly, Sonny Bono's biggest contribution to the U.S. policy is the Sonny Bono Copyright Term Extension Act. Though Sonny Bono was not elected to Congress to help pass this act, he was the initial one to suggest it as a campaign promise when he was running for Congress. Because of his suggestion, the act was passed on and eventually passed by Congress in 1998 due to Bono's suggestion.
This act somewhat directly prevented the missing nine The New Scooby-Doo Movies episodes from being released on DVD. Affectionately nicknamed, the "Mickey Mouse Protection Act," this act disallowed the release of entertainment content into the public domain for without explicit consent from all involved parties for 75 years. Before the act was passed, the guidelines regarding "all parties" were much looser, and content was allowed to be released into the public domain after 28 years if the content was released before 1978.
Put in simpler terms, it appears that since The New Scooby-Doo Movies was released in 1972 and 1973, the episodes would have been fair game for Hanna-Barbera to release as early as 2000 and 2001 (28 years from the original air date). Because of Sonny Bono's Copyright Extension Act, Hanna-Barbera will most likely (unless any of the guest stars change their minds) be unable to release the episodes until 2047, 75 years after the initial air date. (Is that a crazy coincidence or what?)
You can read more about Sonny's political contributions and the Act on the Wikipedia article.