The open-source visual novel engine Kirikiri (used by Fate/stay night, among others), includes an application called Loop Tuner. Loop Tuner is an application built for setting up precise loops in OGG music files. And did I mention it's free? But it's in Japanese, so if you're not confident in your ability to figure out a foreign-language GUI, this guide is for you.
First, go to the above link and grab "kr2_232r2.zip" from the downloads section (Ctrl+F if you need to). Save and extract wherever.
Once you've gotten the archive extracted, browse to "kirikiri2\tools" and run "krkrlt.exe." I won't walk you through opening an audio file, it's the same as any Windows application.
Once your OGG is loaded, your window should look something like this:
It looks a lot like Audacity (which stole its interface from an old program called WaveEdit).
"But Wolf, where'd that arrow come from?" That's a loop. You create one by pressing the button marked A in the screenshot above. It will place two red dashed lines, indicating a loop. Drag the lines around to set up a preliminary loop, keeping in mind that the start of the loop redirects to the position at the end of the arrow. (If you create two or three by mistake, the X button will let you delete them. That's part of the Kirikiri-specific functionality that won't do anything in RPG Maker.)
The play controls also work a lot like Audacity. There are four buttons, Stop, Play from Beginning, Play From Cursor, and Ignore Loops. Don't worry so much about the last one. "Play From Cursor" means playback will start form wherever you last clicked in the waveform, and any loops set will be observed. Keep messing with your loop until it sounds about right.
When you're close (off by about a second or so), double click the loop arrow (it will turn green) and another box will pop up.
This window lets you fine tune your loop by matching the start and end points (blue and red). The buttons at C let you nudge the waveforms left and right (you can also drag the waves themselves). The buttons at D will play the first few seconds before and after your loop. Try to overlap the waves as much as possible, and use the playback buttons to preview your loop. You can also zoom out if you need to.
A perfect loop looks like this:
Getting your song to loop this perfectly probably won't happen unless it was very specifically encoded to do so (this one is from a different file). It's most important to match up the red part right after the loop as much as possible. So long as you don't loop in the middle of a peak, the human ear won't be able to tell the difference. When it sounds about right, press OK, then hit the Save button.
NOTE: Save does not write loop points into your OGG. Loop Tuner is freaking magical, but not that magical.
There's one more step, and it involves opening the ".sli" file Loop Tuner just generated in the folder with your OGG. Don't be afraid, it's only a text file with a fancy extension:
The only two values that matter for RPG Maker purposes are the "From" and "To" values. "From" is the end of the loop. "To" is where the loop redirects. So your LOOPSTART value is the exact number in "To", while your LOOPLENGTH value is "From" minus "To", the length of the loop. Use Calculator to save yourself some headache.
How you embed the values is up to you. I prefer using Winamp's media info editor, because you can just add them like this:
Save the tags and put the OGG file in your game's "Audio\BGM" folder. So long as you entered the right numbers, you now have a 100% compatible looped OGG to rival the RTP.
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