Please follow the steps below to resolve the issue:-
Launch bixby>>tap three dots on right top corner>>settings>>voice response>> tap "always".
Further, launch Bixby by pressing and holding bixby key>>Keep holding the key>>give command to open spotify
Repeat the process to open music file in bixby.
Team Samsung Members
I found the solution if you still need it. The best way to do this is by pressing bixby button once. After you did this press the three lines in the left corner of the bixby home screen. After you done this press the settings icon. You will be redirected to Bixby Voice Settings. This is the final step: scroll down to All Devices category and tap on privacy, you'll have to scroll down again and you'll see "Bixby country/region", press it and select the United States. After you set this you just say Play Music to Bixby. You can handle it from now on. Please let me know if you've got it.