This should be what you need:
$AccLst = new-object System.Security.AccessControl.DirectorySecurity
$AccLst = Get-Acl "C:\MyFolder"
$AudRul = New-Object Security.AccessControl.FileSystemAudRul("everyone","Modify","ContainerInherit, ObjectInherit", "None", "success")
$AccLst.AddAudRul($AudRul)
$AccLst | Set-Acl "C:\MyFolder"