"No sound" is a sort of vague comment... does this mean "very quiet" or does it mean "absolutely nothing?"
Any standard stereo plug should work just fine with an Archos 5IT. If it doesn't work... try another set of speakers. If you're getting sound from headphones, the issue is almost certainly with the speaker (or the speaker cable!) you're using.
In my own case, I have a set of iHome speakers, which I absolutely love. I put my Archos onto a battery-dock or mini-dock, plug the iHome speaker's "USB POWER" cord into the USB port on the dock, plug the speaker stereo plug into the Archos stereo jack, and it's like having a full stereo system. I LOVE the sound I get from these little iHome speakers... by far the best at this size range I've ever seen!http://www.ihomeaudio.com/iHM79BC/
There are two drawbacks I've found to these, however...
1) Twice, I've personally seen the right-channel speaker in one of these speaker sets stop working after being put through an airport "magnetometer" station. If you travel with them, don't carry them in your carry-on, put them into your checked baggage. Not sure what it is that failed, but at least I know iHome knows about the issue, and (hopefully) has addressed it with a design revision.
2) These use a small rechargable battery. Eventually, as with all batteries, this will stop taking a charge. The device can be disassembled and reassembled easily enough, if you have fine tools, and a new battery can be installed, but it's not a trivial operation... roughly akin to replacing the battery in an Archos, except there are two speakers, so you'll need two of the little batteries.
Those two drawbacks aren't enough to make me not recommend these, though. They're GREAT, and perfectly paired to my Archos.. or, for that matter, to my netbook. In either case, they give me great sound through the provided "headphone jack."
I suspect that your problem is not with the Archos, but with the speaker set you're trying to use. The Archos headphone jack is only there to work with headphones, so if you use external speakers, they need to be designed specifically to work on a headphone jack (as opposed to a "line out" jack or a "speaker" jack, like, say, most computer sound cards have).