Does the sound mentioned in the noble Hadith concerning the emission of wind ("if you hear a sound or smell an odor") refer to the sound heard by someone next to the person, or the sound heard by the person himself only?

Certainty is not dispelled by doubt. When you perform ablution (wudu') correctly, you are certain of your purity. This purity is not removed merely by doubt or obsessive thoughts (waswasa), but rather by the certain occurrence of an impurity (hadath), such as smelling an odor or hearing a sound (of passing gas). It is not removed merely by stomach gurgling. However, if you hear the sound from someone next to you, then the invalidation of ablution is even more certain.