Maybe this can be confirmed by TheSource, but just skimming though the Sinobi site revealed that the pre-orders for Wii Music in Japan aren't anything to write home about.
「しかし、予約状況も含めた市場の反応は渋く、今ひとつ盛り上がって来ない。」
Another odd thing I noticed was that when the Wii ads fired up on TV in Japan last night, I was thoroughly expecting to see heavy Wii Music advertising, but instead just got the same old Wii Fit ad (yes, Wii Fit is very, very heavily pushed in Japan via TV ads). No Wii Music ad, at least not during that break.
Obviously if the title turns out to be a strong long-term seller then Nintendo will once again be laughing all the way to the bank, but...
PSN - hanafuda