Yes it is sound money as every secure token thats good to measure a value with. That doesnt mean that bitcoin is a replacement of any other type of money. Usualy its the bad money that is so numerous and so widespread that it literaly wash out the market of other currencies, like fiat does.
Besides that Bitcoin is way to slow as an replacement for money.
bitcoin is being use as a cash money in the case if someone exchange their bitcoin to the fiat money but I think bitcoin has the ability of buying objects and saving money and to buy anything you want through online payment so I think bitcoin has provide an ease for those who cannot go to the markets and cannot spare time for markets so they love to use bitcoin online payment service.
Bitcoin sometimes I think is more than just a cash or a stored money at bank. I am impressed by the bitcoin features, without going to any bank you can save your money i.e. bitcoin.
There are rules and regulations of all the financial institutions and the formalities which may not be sometime possible to be fulfilled by all at a time and in such a time if you have bitcoin in your wallet you will know the ease that you are provided with the bitcoin. If it is transferring money to someone or paying any online bills.