Launch Media, the Santa Monica Internet music company, is expanding to Japan through a partnership to be announced today with two Internet heavyweights. Softbank Publishing, a subsidiary of Japanese Internet investor Softbank Corp., and Yahoo Japan will join Launch in creating a joint venture called Launch Japan, a music Web site with songs and editorial content in Japanese. Softbank Publishing will own 50% of the new company, while Launch will have a 30% stake and Yahoo Japan will own 10%. The other partner, television and radio broadcaster Tokyo Broadcasting System, will own the remaining 10%. Launch Japan will be based in Tokyo with day-to-day management from Softbank Publishing, which also operates ETrade Japan, E-Loan Japan and 13 other online companies. Softbank Ventures was an early investor in Launch Media, whose stock dropped 13 cents to close at $19.88 in Nasdaq trading Wednesday.