if the cones are good quality ie not crappy paper
Paper cones are not all crappy, many excellent Hi-Fi speakers had paper cones, as they are 'doped' to produce a controlled breakup as frequences rise and in many respects
layeredpaper has excellent characteristics - self-damping - rant-over about misconceptions - horses for courses.
Depending what the cone material is, and what the size and frequ range of the driver is ??? (this is Tech Advice, right?)
and you can get at it, try to repair with similar, but keep it thin...
Paper, if the edges go back together, then some latex glue (be sparing else you'll upset the cone's breakup mode), with a little reinforcement, something like tissue paper, crinkle it up first and then flatten it back, maybe puff some fine water mist over it on a dry day first, and do both sides if you can get at them
If there's a clear hole with missing material, pull it back together as much as possible and try soaking some egg-box cardboard like paper mache, thin, don't hack it all a round, water down the latex first, not the heavy stuff carpet fitters use without thinning. Put backing tissue on one side, then a little mache, then finish the other isde with a laminate of tissue paper
If the cone is Butyl, Vinyl, or a composite laminate of synthetics, then mayeb you could try a RTV silicone? Again keep everything thin, don't plaster too much about unless looks like it needs it..
If you have any rubber glue, that might prove strong enough too