I do believe that Bitcoin is sound money as it's value can increase a lot in its lifetime. It is a cryptocurrency that is booming and has a very bright future ahead of it. I can see a future where cryptocurrencies will dominate the currency market and paper money will be non-existent. It just makes more sense to use digital money and leave paper alone.
Of course, Bitcoin can be classified as both an asset as well as money, a currency.
If you can spend it buy the things you need, then perhaps that is enough for it to be considered sound money. But this money, take note, is not the money that we used to know. Because this is a sort of an advance money, a digital or virtual one.
Here's the catch: gold was used as a money long time ago, but when the time came when the value of gold went up so high, it ceased to become money anymore. Instead, it became an asset, a precious one. People kept it away; they don't spend them anymore until such time another kind of money totally replaced it. The same with silver. Bitcoin has come to that age, I guess. The value is so high already that I think it is much wiser to keep them rather than save them, earn them rather pay them.
I agree with what you have to say, this is exactly how I see it too. It will be interesting to see what happens to bitcoin when people start spending them like crazy and don't hold onto them. A bit like in Japan where you get negative interest when you deposit your money into your bank account. This ensures that money is kept in circulation.