I want to change the sound of outgoing mail on my Macbook but I seem not have permissions and have yet to find a work around, any ideas?
Catalina and later have a read-only System volume filesystem layout. Mail.app resides on that volume (at /System/Applications/Mail.app). You will need to remount the System volume as read-write in order to modify Mail's bundle.
EDIT:
To elaborate, here are the steps:

Reboot into the Recovery OS (hold CommandR during boot).
In the Recovery OS, open Terminal (Utilities -&gt; Terminal).
In Terminal, run: csrutil disable
Reboot into Single User Mode (hold CommandS during boot).
Remount the root directory as read-write: /sbin/mount -o rw /
Make your modification to Mail.app (e.g., cd /System/Applications/Mail.app/Contents/Resources &amp;&amp; mv &quot;Mail Sent.aiff&quot; &quot;Mail Sent-ORIGINAL.aiff&quot; &amp;&amp; cp /path/to/YOUR_NEW_SOUND.aiff &quot;Mail Sent.aiff&quot; &amp;&amp; chown root:wheel &quot;Mail Sent.aiff&quot; &amp;&amp; chmod 644 &quot;Mail Sent.aiff&quot;)
Reboot: reboot

Replace /path/to/YOUR_NEW_SOUND.aiff with the actual path to your desired AIFF file.