Why can't I run ausearch (part of auditd) remotely over SSH?

Since you logon as a normal user you might not have /sbin in your $PATH, which means that ausearch might not be found. To try this, specific /sbin/ausearch manually in your command line. For some commands you also require ssh to aquire a tty, you accomplish this with the -t flag, so to try this out, type:

ssh -t myserver "sudo /sbin/ausearch -k my_key"

To fully emulate a logged in session you can also call sudo with the -i flag, and then you can probably omit the /sbin (since it worked in your logged in session), as this:

ssh -t myserver "sudo -i ausearch -k my_key"

Instead of the -t option to SSH, try adding --input-logs to the command (see the man page)

Ref: https://bugzilla.redhat.com/show_bug.cgi?id=1032706#c13