Can you buy just the pedals ? We bought this for Christmas and one of the pedals is no longer working .

Usually the failure point is either at the end of the cable where it connects to the main unit, usually this is a common 1/4 inch phono plug that can be cut off and replaced easily at any pro audio music store that offers repairs as it's the same as every electric guitar, keyboard, amplifier, etc. Or it's a bad connection at the other end where it connects to the pedal. If so, they can usually repair it as well by cutting off a bit of cable containing the bad connection and resoldering the pedal. If yours has a phono plug at both ends they can replace the plug. Better yet replace the entire cable with quality cables. The cables and termination ends are where they save money on the cheaper units as smaller junk cables work the same as the quality cables do, they just don't last long do to the rough treatment and plugging and unplugging of cables. Electrically speaking a sensor input wire (non-power) the diameter of a hair with 1mil thick insulating jacket works the same as wire the diameter of my thumb with 50mil thick insulating jacket, exactly the same for all intense purposes. Except longevity.

Pls clarify. i can attach an external amplifier / speaker through the headphone jack?

YesIf there is a headphone jack, whether it be a 1/8th inch ( 3.5mm ) or a full size 1/4 inch stereo phono jack you can use it as your audio signal input into an amplified speaker (PA speaker) or to a separate amplifier that feeds a speaker. You can use a "Y" splitter at the drumset and run both, headphones as well as a PA speaker. I use a 2 channel mixer so I have more control over the volume of each that you won't get with just a "Y" splitter. You'll at least want a volume control on your headphones if nothing else. Most all of these have a separate dedicated audio out OTHER than the headphone jack. Usually it's in the form of a mono 1/4 inch phono jack, just like found on an electric guitar, or sometimes as simple RCA jacks, either a single RCA, mono audio out, or possibly 2 RCA jacks stereo audio out, one usually white in color for Left and one usually red in color for Right. Keep in mind, you may have 2 RCA jacks but one jack may be an input if your unit has recording and mixing capabilities. That way you can play music in and loop it out with the addition of that awesome beat you're putting down with your drumset there. If there is a stereo output it's usually on the upper-end professional drum machines and not a practice pad and such. I just took a more in depth look at this one and it seems the headphone out is all you get. But you can most definitely go from it to an external amp or powered PA speaker. Just remember you'll control it through your headphone volume , i.e. headphone volume all the way down = No audio being sent to your external speaker = NO SOUND FROM SPEAKER. Also, being drums make sure your external speaker can handle bass and even the expensive electronic drums like my VDrums sound like crap unless you go through Equalizers and Digital Sound Processors and be prepared to spend HOURS and HOURS tweaking it. It's never a plug and play with perfection. Where's the fun in that? A 2 channel mixer is around $35 with both 1/4" phono AND RCA jack on each channel. This helps when your drums have RCA output only and your PA speaker has the common 1/4" phono.....only of course. Also, remember some Powered speakers have a headphone jack on them as well and some have their own separate headphone jack volume control knob. You can plug your headphones in there if so. If it's close enough to do so. Just remember, like your speaker for your drums make sure your headphones can handle the lower frequencies of a bass drum and similar. Usually the larger the drivers the better. Dollar store ear buds will last about 2 minutes, if they play at all. Usually calls for over-the-ear full size headphones like you see in a studio.