Dont mix channels in memory about 64doom

[jnmartin84]

            next_mixed_sample += (ssmp1 | ssmp2);
80001724:	00641825 	or	v1,v1,a0
80001728:	00c33021 	addu	a2,a2,v1
        for (size_t ix=0; ix<NUM_VOICES; ix++)
8000172c:	24420024 	addiu	v0,v0,36
80001730:	1447ffcf 	bne	v0,a3,80001670 <I_MixSound+0x50>
80001734:	00000000 	nop
    for (size_t iy=0; iy < (NUM_SAMPLES << 1); iy+=2)
80001738:	254a0002 	addiu	t2,t2,2
        }

        *((uint32_t *)&pcmbuf[iy]) = next_mixed_sample;
8000173c:	ad860000 	sw	a2,0(t4)
    for (size_t iy=0; iy < (NUM_SAMPLES << 1); iy+=2)
80001740:	154effc9 	bne	t2,t6,80001668 <I_MixSound+0x48>
80001744:	258c0004 	addiu	t4,t4,4