I'm assuming this is a GD series, ( 2nd gen) i30 with a GD-01 head unit. I suspect they have been known to fail but then so has the occasional FD series units.
Again assuming that the unit is not muted and all functions have no sound I'd say find a working used unit and swap it out or maybe there is a later version stereo that could be a handy upgrade . I've never touched a GD so likely someone will have some sound advice.
The fault is likely internal, could be something simple like bad solder joints but It would not be a viable proposition to have anyone look at it if it needs removal and dismantling.
Just my opinion.