After some trial and tribulation, I was able to find all .msi files in a given directory and install them.
foreach($_msiFiles in
($_msiFiles = Get-ChildItem $_Source -Recurse | Where{$_.Extension -eq ".msi"} |
Where-Object {!($_.psiscontainter)} | Select-Object -ExpandProperty FullName))
{
msiexec /i $_msiFiles /passive
}