I installed latest soundmax drivers (which are supposed to use less CPU)- it helped a bit.
I enabled Asus AI NOS (program which automatically overclocks CPU when needed)- it also helped.
Now those 2 above work together- and freezes during explosions reduced to 1/3 or even 1/4 of their previous time (now they are hardly noticeable).
I'm not going to buy a sound card only to improve SH3 performance

The one on the motherboard is OK- works fine in other applications, the chopiness comes rather from not-too-smart sound system made for the game which makes a soundcard work hard (I guess that making all those sunds .wav files was not the best possible solution

)
I'd rather buy a faster CPU (or overclock the one I have).
And I'll still be looking for a software solution for this problem- i'll ask some geeks...