I exported my Voice Control Custom Commands but how do I open the file to see a list?

You need to extract the elements of that XML plist to get at the actual custom voice commands. If you copy and paste the following AppleScript code into Apple's Script Editor, click the compile (hammer) buitton, and run, it will present you with a dialog list of your custom commands (more or less).

use framework "Foundation"
use AppleScript version "2.4" -- Yosemite or later
use scripting additions

property NSArray : a reference to current application's NSArray

set plist to "$HOME/Library/Preferences/com.apple.speech.recognition.AppleSpeechRecognition.CustomCommands.plist"

set custom_commands to paragraphs of (do shell script "plutil -p " & plist & " | awk -F '\"' '/0 =>/ {$1=$1;print l;l=substr($2,1,length($2))}'")

if (count of custom_commands) = 0 then return

set custom_array to (NSArray's arrayWithArray:custom_commands)'s mutableCopy()
custom_array's removeObject:""
display dialog (custom_array's componentsJoinedByString:return) as text with title "Custom Voice Commands"
return

It appears to be an XML Preference List. I don't see any code in my test export.

There does seem to be some encoded blob of data, but the list of commands are just property list key/value pairs.