Bluetooth is used for transmitting or receiving data digitally. If you have a USB Bluetooth dongle connected to a PC or if it is built in, then only on the receiving end can it be considered audio because of the electronics that convert that data in to sound, which is then outputted by speakers or headphones.
A USB Bluetooth transmitter cannot receive a signal from a sound card, because there is no link between it and the sound card. A sound card or motherboard audio can only output audio signal via speakers or headphones connected directly to it's analogue connections, or digital data via SPDIF optical or coaxial ports.
For a Bluetooth device to receive a signal from a sound card, then it would have to be the type of transmitter that has 3.5mm input, which can then be connected to the sound card 3.5mm output.
While such transmitters are great for adding Bluetooth to a TV that does not have it built in, for example. Using one as a way to get Bluetooth headphones to work with a sound card, then the merits of doing that are considerably less; to the point where you might just as well use a Bluetooth USB dongle instead.
A Bluetooth transmitter with 3.5mm analogue input; while that takes the signal received from the sound card, it then converts that sound signal into digital data, which is then compressed so it can be sent via Bluetooth radio wave. All that negates the value in connecting such a Bluetooth transmitter to a sound card especially a more expensive one.
If there were issues with using a Bluetooth dongle, like Windows mucking up drivers etc, then there would be more merit in using a Bluetooth transmitter with 3.5mm input.
