YOU\u2019VE probably heard musicians like Neil Young complain over the last few years about the sound quality of digital music. But thanks to a new generation of so-called audio enhancement apps, your music\u2019s sound quality doesn\u2019t have to entirely depend on your smartphone. LouderLogic, free on iOS, is perhaps the most impressive audio enhancement app I have used. LouderLogic functions just like the iPhone\u2019s Music app and looks much like any other music-playing app, except for a large green button labeled \u201cALX\u201d that turns on its \u201cAudio Level eXtension\u201d technology. ALX makes tones sound much warmer and vocals seem brighter. It gave movies theater-quality sound, even on headphones. The ALX button automatically delivers this effect, but users can manually adjust the sound quality with audio equalizer controls when the phone is turned sideways. Obviously, ALX works better with certain tracks. The app\u2019s design is a little garish, and there are some pop-up ads in the free edition. But an ad-free version is available for $2. On Android, an equivalent app is DFX Music Player Enhancer, which is also free. The app calls itself the first to bring \u201cprofessional audio quality\u201d to Android devices, and says that, like LouderLogic, its goal is to restore some of the \u201clost natural depth\u201d of music recordings. The DFX Player works as advertised. It makes music sound livelier, the bass notes warmer, than through a typical Android music app. You can also precisely adjust audio levels and choose settings to suit different types of music. On the downside, the app\u2019s interface is a bit clunky and not as polished or easy to use as it could be. Headquake, a free iOS app, might be the most impressive. Headquake uses a special-effects engine called Sonic Emotion to give sounds a 3-D quality. It tries to manipulate a standard stereo music track so the music sounds as if it is coming from different parts of the room. Like LouderLogic, the app synchronizes with the music in your iTunes collection. But it includes a large \u201c3d\u201d button, which activates a whole new spatial dimension to sounds: You can actually get a sense of, say, the singer in a band standing in front of you while the guitar comes from the side. You can control the effect by sliding icons on the screen to try to centralise the location of the sound and by adjusting equaliser levels for bass, midrange and treble. But Headquake\u2019s special effect doesn\u2019t work well for all songs. With hard rock, for example, it makes midrange sounds mushy instead of clean, and singers can sound like they\u2019re singing in a toilet bowl. For the right track, however, the effect is startling. And Headquake is free, so there\u2019s nothing to be lost by giving it a whirl. LouderLogic, DFX Player and Headquake may not appeal to audiophiles because the processing they apply to music has such a bold effect on its sound. On iOS, a slightly more subtle and gentle effect is offered by the free Audyssey Music Player app. The app\u2019s main trick is tuning its behavior to suit your headphones using profiles. Many of the profiles are free, but to get certain high-end profiles like Beats by Dre, you have to pay a few dollars in-app. If all this talk of advanced music processing makes you uncomfortable, simpler audio enhancement is possible with equalizer apps like Equalizer by Audioforge ($3 on iOS) and Music Equalizer by PerfectionHolic on Android (free).