Duke Ellington said about arranging, "Always write for your players." In other words, if you write a horn line you know your horn section can play well, then your arrangement will be well-played and your music will sound good, kinda by definition. The same is true when creating audio for mobile games: you have to write for your players, which in this case is usually a speaker the size of your thumbnail.
First of all, this means no bass—I mean, none, nada, fuggedaboutit. Don't write music that gets its power or groove from a deep funky bass line, because nobody's ever going to hear it. Snap of the snare drum—yes, boom of the kick drum— nnnnnot so much.
The same thing goes for sound effects. When making a car crash, pick a sound that has a lot of high end, then EQ the bottom right out. Applying a highpass filter at around 250Hz and maybe adding a bump around 3kHz will prevent low-end rumble from distorting the speaker, and make the sound pop where the speaker is most sensitive.
Figure 6. Waves L1 is an extraordinarily useful plugin that helps audio designed for cell phone speakers play loud and clear.
Another thing you're going to want to do is apply a liberal dollop of L1 to almost every sound you make (see Figure 6). L1 is a compressor/limiter plugin from Waves that squishes the peaks and raises the overall volume of a sound file; it is used to fill up as much of the available digital range as possible, as shown on the right side of Figure 6.
This does two good things. First, it gives you a strong signal so that your sound will play loud and clear on the tiny speaker, and second, it gives your downsampling and compression algorithms as much meat as possible to chew on while they decimate your audio data to save space. When processing a high-resolution, CD-quality sound effect down to an 11kHz, IMA 4:1 compressed sample, a soft sound like the one on the left side of Figure 6 is going to come out sounding a lot like static.
This doesn't mean you can't make good sounds on cell phones; you just have to write for your players. For mobile music, melody is king. For sound effects, well, there is one category of sound that cell phones are specifically designed to produce well--vocals. Voice-overs, screams, laughs, sneezes, sighs, grunts, groans, pretty much any sound a mouth can make is guaranteed to come through a cell phone speaker with some sort of fidelity. Here's a good example.
Bob is a game that uses a number of interactive audio techniques to create a varied soundtrack. The music mix changes as the player progresses through each level, the frequently played bounce sound effect is varied depending on how fast the sprite is falling, and Bob, being a loud happy guy, says things like "Yeah!" when he gets a power-up, "Whoa" when he falls too fast, and "All right" when it's time to play. And if these vocals sound a little familiar, yes, it's true, I admit it ... I am the voice of Bob.
Figure 7. The whimsical Bob is one of Danger's best selling games. (Click image to play movie clip.)
Notice how the music mix changed as we played the game. The bass line got faster, the electric piano came in, etc. Each level has six or seven different combinations of instruments and tracks that flow from one to the other as you continue forward, with a full mix playing when you complete the level. That way, the music is always changing, and there's a little reward at the end.
Figure 8. This view of the tile editor shows a section of one level with the bits for "Audio Mix #5" shown as a blue line.
This variation is accomplished by setting bits in the level map using an editor as shown in Figure 8. Each black dot represents some sort of special tile bit (bounce, enemy, heart, etc.) and the bit for "Audio Mix #5" is highlighted in blue. Therefore, when Bob passes through that thin blue line (and he always will at some point, because the game is a side-scroller), the music will change to mix #5. Later, if you don't fall into the bottomless pits at mid-screen, you'll hit the next audio zone, shown in Figure 8 as a line of black dots in the sky at the right, and then Mix #6 will play.
Figure 9. Why is this soundtrack called "the Secret Yanni Technique?" Click the image to play the movie clip.
Now we get to one of my favorite tricks of the trade, which I like to call the "Secret Yanni" technique for reasons that are too arcane to explain here. Basically, the idea is this: video games are not movies; there's no concept of "sync to picture" because you can't always predict when a sound effect will be played. But knowing this, you can use serendipity to your advantage, as we did in Bob with the sound effects for bonus points.
In order to save space, we wanted to play a few MIDI notes every time Bob picked up a heart. MIDI is an excellent (and in many cases, the only) choice for making sound in the mobile environment because it is an extremely efficient and flexible technology. But the question becomes, "Which notes do you play?" There's no way to know when Bob is going to pick up points, so there's no way to know when the bonus sound will play or how it will fit with the music. You want the effect to be "Yay!" but if the notes clash with the music, the effect is going to be more like "Ouch!"
The answer? Modal music. You restrict your soundtrack to a single mode, no key changes, no fancy chord progressions. Just keep it simple, baby—and in many cases, this is a good way to write background music anyway. For Bob, the chords are mostly ii-IV-V-I in C, and then I picked notes for the bonus points that fit the mode, using a pentatonic scale consisting of C, D, E, G, and A. If you want to get really clever, you can have the notes play at the same tempo as the music, or some polyrhythmic fraction of it. Arranging things in this manner will produce sound effects that almost never clash with the music, and sometimes will even seem to be part of it. (Click on Figure 9.)