Transpose Sound (Pitch)

Best way is to attache a score, else we can only suppose. Maybe you've added a "8va treble clef" instead of an "treble clef"? Or you're using a transposing instrument?

If you are writing for an electric bass or contra bass, it does that automatically if you use the existing instrument defined in MuseScore. Attach a sample score with the problem.

My best guess is you didn't actually add an electric bass instrument but added, say, a piano. If you had added the bass directly it would have got the transposition automatically, either by setting the transposition or by using an octave clef. Or maybe you let it add the octave clef, then removed it. Either way, you now would need to set the transposition yourself. Right click the staff, go to Staff/Part Properties, set the transposition to down an octave. Now you'll hear what you expect - notes sounding an octave lower than written, as basses do.