You can use Voice Control (available only on iPhone 3GS) to control music playback on iPhone.
NOTE: Voice Control isn’t available in all languages.
Use Voice Control:
Press and hold the Home 
button until the Voice Control screen appears and you hear a beep. Then use the commands described below to play songs.
You can also press and hold the center button on the iPhone headset (or the equivalent button on your Bluetooth headset) to bring up Voice Control.
Control music playback:
Say “play” or “play music.” To pause, say “pause” or “pause music.” You can also say “next song” or “previous song.”
Play an album, artist, or playlist:
Say “play,” then say “album,” “artist,” or “playlist” and the name.
Shuffle the current playlist:
Say “shuffle.”
Find out more about the currently playing song:
Say “what’s playing,” “what song is this,” “who sings this song,” or “who is this song by.”
Use Genius to play similar songs:
Say “Genius,” “play more like this,” or “play more songs like this.”
Cancel Voice Control:
Say “cancel” or “stop.”