Yeah, you'd be surprised how good even less than 5 watts can sound in the right enclosure.
shine, you may just be working with crap audio... Have you played this back on a decent sound system to find out if it's the clip itself? re-recording the MP3 may help, it could just be a really low bitrate one, no matter what you do it's gonna sound like crud. Most modern cell phones can play back 192khz mp3 files, some better.
Audio source is normal quality, 192kbps mp3. But I re-encode it to 128kbps, it's quite enough for phone.
The issue related with audio spectrum of a specific audio clip.