I may sound a bit newbish, but I'll ask anyways. What is the best file format for BGM's,BGS',ME's and SE's? And how can I reduce their file size ( My Audio folder already eaten 200MB and I don't plan to have a 1GB game you see ... ) All joking aside, thank you.

MIDI are generally the lightest format, since their weight it's around 30-50Kb... I reccomend you use it for BGMs and MEs
As regards SEs and BGSs, I actually have no idea.
In my opinion, you'll have to reduce the use of MP3 files, that are generally too large.
However, for MIDI-manipulating, you can use a program like TuxGuitar (freeware) or GuitarPro (not freeware at all...)
With that, you can import and modify your music, if you know something about notes, pentagrams, and so on...

Jens

EDIT: I've heard something about .ogg files... the standard format of SEs in the standard RTP... they seems to be light enough...
MP3 are a serious problem, but if you plan to use them, be sure their Kbps rate to be lesser than 128. It means: low quality but low weight.
I think a 200 Mb folder is something just too big...

I see. I'm planning on not using MP3's at all since they really are the prime suspects of making a game project too large, so I'm wondering if there is any way I can reduce their size, and still produce high quality audios. I've heard about MIDI's for BGMs, but not .OGG files for SE's so I think I'll experiment with that.

.ogg files seem to be the smallest file size format you can convert mp3 audio files to. The only issue with this, is that when played in RMXP, ogg files must be loaded entirely before the sound file plays. If you plan on using them, it would be best not to use them for long audios, since there will be a noticeable delay. I was using them for all my music at one point which was a very bad idea, as I would have lag before entering battles/new maps/BGM changes.

If you don't like midi quality, I would stick with mp3s and use a program like wavepad or something to save them with a higher level of compression. I would only use ogg files for sounds effects or MEs, but since those files are typically very small to begin with, it's not extremely necessary. I hope this information helps

I was using .ogg files in my game, loving how they still sound the same but take up no file space at all, but then I realized that they do need to load completely before anything else happens, and that was creating some serious lag-time in my game. I put headphones on, and the quality was like sh*t. SO I'll just be using mp3 files in my game from now on. If someone has a problem with downloading an RMXP game with 200 +/- MB to it because of great audio, then I'll give them a copy without the audio in it xD

Thanks everyone. I have been enlightened alot by informations you've shared. NowI have the idea to maximize the use of audios. Thank you.