check out the app "Narrator" (Mariner Software) - I've just bought the app for a different purpose (correct pronunciation) - but regarding to the description of the app it should solve you're problem too

It's possible with the say command in a shell, not with the AppleScript say command.

Info for the AppleScript say command:

• you can stop the speech of say command from the same script until the script run, not after that the script exit.
• Example:

say "I want to recreate macOS's built-in Text To Speech" waiting until completion no
delay 0.5
say "" with stopping current speech -- this stop the first say command of this script
delay 1
say "Hello"

This script use the say command in a shell to speak the contents of the pbpaste command (the clipboard), and it put the PID of the say command to a persistent property:

use AppleScript version "2.4"
use scripting additions
use framework "Foundation"
use framework "AppKit"
property this_say_Pid : missing value -- the persistent property

if this_say_Pid is not missing value then -- check the pid of all 'say' commands, if exists then quit the unix process
    set allSayPid to {}
    try
        set allSayPid to words of (do shell script "pgrep -x 'say'")
    end try
    if this_say_Pid is in allSayPid then -- the PID = an item in the list
        do shell script "/bin/kill " & this_say_Pid -- quit this process to stop the speech
        error number -128 -- quits the AppleScript
    end if
end if

-- Back up original clipboard contents:
set savedClipboard to my fetchStorableClipboard()

-- Copy selected text to clipboard:
tell application "System Events" to keystroke "c" using {command down}
delay 1 -- Without this, the clipboard may have stale data.

-- Speak the clipboard:
--  pbpaste = the contents of the clipboard , this run the commands without waiting, and get the PID of the 'say' command 
set this_say_Pid to do shell script "LANG=en_US.UTF-8 pbpaste -Prefer txt | say > /dev/null 2>&1 & echo $!"

-- Restore original clipboard:
my putOnClipboard:savedClipboard

on fetchStorableClipboard()
    set aMutableArray to current application's NSMutableArray's array() -- used to store contents
    -- get the pasteboard and then its pasteboard items
    set thePasteboard to current application's NSPasteboard's generalPasteboard()
    -- loop through pasteboard items
    repeat with anItem in thePasteboard's pasteboardItems()
        -- make a new pasteboard item to store existing item's stuff
        set newPBItem to current application's NSPasteboardItem's alloc()'s init()
        -- get the types of data stored on the pasteboard item
        set theTypes to anItem's types()
        -- for each type, get the corresponding data and store it all in the new pasteboard item
        repeat with aType in theTypes
            set theData to (anItem's dataForType:aType)'s mutableCopy()
            if theData is not missing value then
                (newPBItem's setData:theData forType:aType)
            end if
        end repeat
        -- add new pasteboard item to array
        (aMutableArray's addObject:newPBItem)
    end repeat
    return aMutableArray
end fetchStorableClipboard


on putOnClipboard:theArray
    -- get pasteboard
    set thePasteboard to current application's NSPasteboard's generalPasteboard()
    -- clear it, then write new contents
    thePasteboard's clearContents()
    thePasteboard's writeObjects:theArray
end putOnClipboard:

It's possible that the first script will not work, if the value of this_say_Pid variable doesn't persist across runs, it depends how the script will be launched. In that case, you must write the PID to a file, so use this script:

use AppleScript version "2.4"
use scripting additions
use framework "Foundation"
use framework "AppKit"

set tFile to POSIX path of (path to temporary items as text) & "_the_Pid_of_say_command_of_this_script.txt" -- the temp file
set this_say_Pid to missing value
try
    set this_say_Pid to paragraph 1 of (read tFile) -- get the pid of the last speech
end try

if this_say_Pid is not in {"", missing value} then -- check the pid of all 'say' commands, if exists then quit the unix process
    set allSayPid to {}
    try
        set allSayPid to words of (do shell script "pgrep -x 'say'")
    end try
    if this_say_Pid is in allSayPid then -- the PID = an item in the list
        do shell script "/bin/kill " & this_say_Pid -- quit this process to stop the speech
        error number -128 -- quits the AppleScript
    end if
end if

-- Back up original clipboard contents:
set savedClipboard to my fetchStorableClipboard()

-- Copy selected text to clipboard:
tell application "System Events" to keystroke "c" using {command down}
delay 1 -- Without this, the clipboard may have stale data.

-- Speak the clipboard:

--  pbpaste = the contents of the clipboard , this run the commands without waiting, and it write the PID of the 'say' command to the temp file
do shell script "LANG=en_US.UTF-8 pbpaste -Prefer txt | say > /dev/null 2>&1 & echo $! > " & quoted form of tFile

-- Restore original clipboard:
my putOnClipboard:savedClipboard

-- *** Important *** : This script is not complete,  you must add the 'putOnClipboard:' handler and the 'fetchStorableClipboard()' handler to this script.