This script allows you to set 5 different killsounds at once. You have 5 different file names to use for your custom audio files(killsounds):
alias CycleKillsounds "IncrementVar tf_dingalingaling_last_effect 0 4 1; Wait 100; CycleKillsounds"
Place the lines in your autoexec file for use. You can mess with the wait number to see what you like, but it does become a bit unstable/inconsistant when set lower than 24. You also need both the alias CycleKillsounds and the actual Cyclekillsounds lines because one says what it does and the other tells the engine to execute it.
Big thanks to Zwip-Zwap Zapony for helping me come up with this oh so long ago.