The previous answers seem either overly complicated, and/or not particularly suited to a batch file.
This simple one liner should place the desired drive letter in variable myDrive. Obviously change "My Label" to your actual label.
for /f %%D in ('wmic volume get DriveLetter^, Label ^| find "My Label"') do set myDrive=%%D
If run from the command line (not in a batch file), then %%D must be changed to %D in both places.
Once the variable is set, you can refer to the drive using %myDrive%
. For example
dir %myDrive%someFolder
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