For shorter sound files, acid assumes that they are loops. If you switch it to one-shot, the tempo should switch to whatever the overall acid's bpm is set to (e.g. 120 bpm normally). However, there are times when it seems to have a default value and there's no way around it, I usually just render the effect as an mp3 through acid and import the newly created sfx into my project. That usually gets rid of that default issue. HTH