After changing the sound acceleration you need to reboot to have it take effect. While you could alter it before playing DD and put it back afterwards, that might get to be a pain. I would try the other games first; if you can not tell the difference, there is no reason to switch the setting back and forth.
In the Dxdiag Sound tabs, it should list the name in the 'device' section (top left). For example, "Name: DirectSound (SB Audigy Wave Out [4000])" for an Audigy sound card. Lower the acceleration in whichever tab corresponds to the way you listen for the game (speakers, headset or both). Presumably that would be the digital audio, unless you play at night and use the headset so as not to disturb other people.
There should not be much, if any, noticeable difference in other games with a lower hardware acceleration. Games/DirectSound would have to do a few more things in software that way, which requires more CPU usage, which in principle could lower game performance. If the games in question are not already suffering from performance issues, then I doubt even disabling hardware sound acceleration completely would make a significant difference.
DirectX is used by games (and some multi-media players) for sound output. The way sound is handled is not related to the save process, so you will still be able to use your savegames.
In general, save games are not tied to hardware or specific hardware settings, so changes to DirectX, drivers or upgraded hardware should not effect them. One exception is Diablo, which stuck some game data in the Windows registry, so without a third party utility saves could not be backed up, moved to a different computer or restored after re-installing Windows.
