MIDI Converter Studio is able to convert multiple files at once. Just add all MIDI files that you want to process either one by one, or by selecting the whole bunch at once. You can use both Drag&Drop and "Add Files..." button.
I also use the MIDI converter software to create audio CDs for choir members to learn their parts. Some of them don't have computers and e-mail to use MIDI files, so with the converter I can give them a CD to play on standard audio equipment.Earl Hughes