???
You say that 1080 fps is the speed of sound, while in the same sentence claiming that 1000 fps is NOT below that. Now, I'm a semi-ignorant hillbilly, but when I went to school, 1000 absolutely WAS below 1080. Was that not the case where you went to school?
But elementary-school ciphering aside, the simple (and easily researched) truth about the speed of sound. It does vary based primarily on temperature, and much less so based on elevation, humidity etc, but on most of planet Earth, it's not only above 1000fps, it's actually above 1100fps.
From
Speed of Sound Calculator
At 90 degrees fahrenheit, speed of sound is around 1149 fps; and depending on those other variables it could vary very slightly. 1000 is below 1149.
At 75 degrees F, it's 1133 fps. 1000 is below 1133
At 60 degrees F, it's 1117 fps. 1000 is below 1117
At 45 degrees F, it's 1101 fps. 1000 is below 1101
At 30 degrees F, it's 1084 fps. 1000 is below 1084
At zero degrees F, it's 1051 fps. 1000 is below 1051
At -15 degrees F, it's 1033 fps. 1000 is below 1033
At -30 degrees F, 1016 fps. 1000 is below 1016
You have to get all the way down to -43 degrees F to get the speed of sound down to 1000 fps.
So yes, 1000 FPS is well below the speed of sound
almost always.