Well, as ridiculous and insane as this question may sound, you’d be surprised to find the answer. No, you aren’t going to sink. Wait! This doesn’t mean you won’t die, obviously. You can hardly compare the super-hot magma with water as the molten lava’s viscosity is way higher than any liquid and hence, sinking is far beyond question.
The probability of falling into a volcano is very less in itself as you might, in fact, already be dead as you near the rim of it either due to the unbearable heat which might cross 1000 degree Celsius or due to the toxic gases that will lead to asphyxiation. So, if at all you manage to jump into a volcano, which sounds terrible in the first place, it would be possible only from an airplane accident. Even airplanes aren’t permitted in active volcanic regions in that case.
In spite of the minute chances of ending up inside a volcano, it’s going to be as hard as hitting a rock. Of course, the body will ultimately burn into ashes without doubt after severe bone injuries. Isn’t it chillingly scary to even read this? Anyway, at the least, let’s not try anything as stupid as going near a volcano, if at all that’s practical.