Hissing sound is normal. Every sound device hisses. AC97 is a codec standard. It does not tell me or anyone a thing.
Since you are using a different driver that you had in Redhat, its the driver you are using in Slackware. You can try setting up ALSA again in Slackware or live with what you got until the next OSS driver version.
The hissing sound comes from the Total Harmonic Distortion or THD although not a lot of devices are measured in N+THD. N+THD is noise added to the THD equation. N+THD is the realistic number that you will hear in the real world. Low numbers of THD will give the least amount of hissing. Computers makes a mess to audio devices so expect near 1% N+THD or more.