Just a wild guess, but they seem way too big, they wouldn't fit the audio ram and you might also run out of ram memory.
Try playing with the malloc setting to free some ram to fit them.
You could also just send them to the low work ram (I don't know by heart the adress, but it's something like 0x20000), but I'm not sure if it will work.
I don't know if it can be stored in the audio RAM either as I've never tried it, so you can also give it a try if you want.
Try to declare it something like :
#define LOW_WORK_RAM (0x20000) //I just typed something, don't copy it blindly as I'm not sure it's the right adress)
jo_sound * blop;
void load_cop_sound(void)
{
blop = (jo_sound*) (LOW_WORK_RAM);
jo_audio_load_pcm("B.PCM",JoSoundMono8Bit, blop);
}