My criterium for "available" is: being able to access the files on the
share.

You can create a test file on the mount like so:

touch /home/username/multimedia/testfile

Then run a bash script to check for the accessibility of that test file every 60 seconds like so:

#!/bin/bash

# Set the full path to the test file between " " in the next line

file="/home/username/multimedia/testfile"

while true; do

    if [ -f "$file" ]; then

        echo "File is available"

        # Your command/s here when the files on the mount are accessible.

    elif [ ! -f "$file" ]; then

        echo "File is NOT available"

        # Your command/s here when the files on the mount are NOT accessible.

    fi

    sleep 60

done