There are a few things to consider when talking about sales:
Single sales grew in between 2009 and 2012, although they fell in 2013. This is when AKB started developing their sales model and when other idol acts started emulating it. Idols may have helped drive that growth. If that's the case whenever the current idol boom dies down (and it will) that may cause further declines in the CD single market.
Japanese digital sales are a different from digital sales in Western markets. Recochoku doesn't allow cross-platform downloads, so if you want to listen to a song on your iPod and your feature phone, you have to buy it twice. In the past five years smart phones have eclipsed both music players and feature phones. This, as previously mentioned, has resulted in the decline of the Chaku-uta market and a decline in digital sales in general. It should be noted that smart phone purchases are considered PC downloads by the RIAJ.
A report on the global music industry projected a 9% revenue decrease for Japan this year. I have a feeling that Frozen becoming such a big phenomenon is going to completely mess with that projection. The Frozen soundtrack may end up saving the Japanese music industry this year.