Right-click the object and select "Edit". Click "More" in the edit window and go to the "Content" tab. Drag a sound file into the object's inventory (the large empty space under "Content"

.
Click the button "New Script". Double-click the script file, delete all text and paste this in:
default
{
touch_start(integer total_number)
{
llTriggerSound("soundfile", 1);
}
}
Replace "soundfile" with the name of your audio file (in quotation marks). If the sound should be continuously looped, replace llTriggerSound with llLoopSound. It will then never stop playing though, until the command llStopSound(); is used.
Example (sound looping for 30 seconds):
default
{
touch_start(integer total_number)
{
llLoopSound("soundfile", 1);
llSetTimerEvent(30);
}
timer()
{
llStopSound();
llSetTimerEvent(0);
}
}
Btw, you can upload any .wav sound into SL, as long as the file is recorded with 44.100 kHz, 16 bit. Use the audio recorder that comes with Windows to convert a soundfile to these settings. It can't be too long though, a complete music title won't be possible (aside from the copyright issues).