Well, as far as my trial-and-error understanding of HL goes, it'll look for files in these places, in this order, top things first:
valvesound
pak9.paksound
pak8.paksound
pak7.paksound
pak6.paksound
pak5.paksound
pak4.paksound
pak3.paksound
pak2.paksound
pak1.paksound
pak0.paksound
Therefore if you put sounds in valvesoundbarney, with the same names as the original sounds, they'll be used instead of the sounds in, say, pak0.pak, and if you make a pak9.pak, it'll overrule pak8 and downwards.
The sound formats need to be as the original, 11,025kHz, 8-bit PCM, Mono.