I don't know how many monsters may be on screen at the same time but you could create multiple copies of the walking sound effect, Walking_1, Walking_2, Walking_3, etc.
Then use Audio > Add Delay Effect and assign a different amount of delay to each.
You can then use Play Audio By Name and Use Choose( "Walking_1", "Walking_2", "Walking_3" ) to assign the Walking sound randomly to each monster when created or spawned.
You will also need to create a condition to assign the delays for each Walking track
Audio> Is Tag Playing > Walking_1 > Add Delay Effect
Audio> Is Tag Playing > Walking_2 > Add Delay Effect
Audio> Is Tag Playing > Walking_3 > Add Delay Effect