From my POV Sony's decline started when then-chairman Akio Morita co-wrote a highly political and far-right leaning book entitled "The Japan That Can Say No". It may be coincidence, but around the time that Morita told the world about his protectionist ideals and disdain for their largest market, the US, Sony products sold in the US started to get really cheap and flimsy.
At the time I was working in television broadcasting, and was shocked by how crappy our newest Sony equipment was. I have less experience with Sony consumer electronics, but heard from others that Sony was doing the same thing in that market.
Needless to say, the racist and imperialist rhetoric that was all too reminiscent of Axis times, coming from the country that had to be stopped with the atom bomb was a major cause for concern. Thankfully Morita's political career went nowhere, and now he's dead.
One thing that we Americans should be aware of is that most Japanese businesses that export to the US and Europe make two different versions of the same products. One version is made in Japan, by Japanese workers and for Japanese consumers. The other version is made in whatever country (currently China) that can make them as cheaply as possible.
When it comes to Sony's stock value I have no opinion. It will be interesting to see how Japan's unique business system deals with a possible foreign takeover. My guess is that Sony isn't in any real peril, and that they'll be around for a long time to come. It's like how Hollywood movies make huge profits, but on paper they appear to be in the red. They're just playing the system.