All forms of sounds which come out of the Shofar are valid. Hence whether the sound was very strong or very light [or sounded hoarse or dry] it is valid. [This applies even if the Shofar itself is able to sound strong sounds and the blower simply is not successful in sounding a strong sound.]
Is the sound valid if its pitch changed during the blow?
Yes. Even if a single blow contains different pitches of sound the blow is valid. Nevertheless some were particular to repeat the blow if its pitch changed during the blow. Practically one is initially to be stringent in this matter to practice to sound a single pitched sound during his blow. Likewise in the event of a changed pitch of sound one is to try to blow a proper length of sound in the changed pitch.
Is the sound of air expelling from the Shofar considered a valid sound?
It is very common that when one is unsuccessful in sounding a real blow one hears the sound of air going through the Shofar. Is this sound valid under the above stated law?
No. A blow is only valid if an actual sound was heard. The sound of air is not considered a sound at all.
A Segula if the Shofar is not blowing properly:
If one is not being successful in blowing the Shofar properly [and he is a good blower and the Shofar is a good Shofar] it is a Segula to turn the Shofar around and whisper into it the paragraph of Vayehi Noam. It once occurred in a community that the Baal Tokeia was unable to blow properly and the Rav of the town, Rav Mechlin, told him to turn the Shofar around and follow this Segula. After doing so the Shofar began to blow properly. The Rav responded that the Satan was standing by the entrance of the Shofar, hence preventing the sounds from being heard [and the Segula removed the Satan from the area].
May one practice this Segula even prior to the first blow? If one is not successful in sounding even the first blow then he may not recite the above Segula, as doing so would be an interval between the blessing and the blowing, and would require a new blessing to be recited. If however there is no other Shofar available in the area and hence bringing a new Shofar would anyways require a new blessing then the Segula may be recited and a new blessing is to follow. One may practice this Segula in between the sets of the Tekios Meyushav and in Musaf. However during the blows of the silent Shemoneh Esrei the Baal Tokeia cannot say this Segula. Rather he is to think the words in his mind.
The Rebbe’s Tekios:
A famed Chassid once asked why the Rebbe’s Tekios on Rosh Hashanah were always so low and came out with difficulty while he heard that in Elul when the Rebbe would blow Shofar for the Rebbetzin the sounds were strong and came out with ease. The Chassid, who was a heavy set fellow, answered “If you had someone like me sitting in the hole of your Shofar wouldn’t you have difficulty blowing it?” Meaning to say that when the Shofar is being blown for the Mitzvah there are evil forces acting as an impediment to the sounds and it is the Rebbe’s job to overcome these impediments and draw down blessing for every individual Jew.
 Michaber 586/6; This Halacha is missing from Admur although see 586/9 and 18
 M”B 586/6; Kaf Hachaim 586/48 based on Gemara R”H 27b
 Levush brought in Kaf Hachaim 586/49
 Ashel Avraham Butchach Tinyana 590; see Piskeiy Teshuvos 586/5; Leket Yosher p. 125; Moadim Uzmanim 1/5
 Mahril Diskin brought in Sheilas Shlomo 2/55
The reason: As they suspect that a changed sound is considered as if the blow has ended and a new blow has begun, and some of the Rishonim hold that it invalidates the sound. [see Ritva R”H 33b; Drashos Haramban p. 240; Meiri; See Moadim Uzmanim ibid; Piskeiy Teshuvos ibid footnote 14]
 Ashel Avraham ibid; Piskeiy Teshuvos ibid
 Ashel Avraham ibid; Meaning if one sounded a Tekiah of 2 Terumatin and the sound changed then he should make the remaining Tekiah at least 3 Terumatin on order so it is valid without needing to join the previous pitch of sound.
 Piskeiy Teshuvos 590 footnote 22
 M”A 585/11; Mateh Moshe 808; Mahril; Kaf Hachaim 585/48
 This story is brought in Mateh Moshe ibid [student of Mahrshal] in name of one of his teachers. This story occurred prior to the 1600’s during the lifespan of the Mateh Moshe.
 Degul Merivava 585 on M”A ibid; brought in Shaareiy Teshuvah 585/7
Why is this not considered a matter relevant to the blowing? As this Segula is not a definite cure to the problem. [ibid]
 Shaareiy Teshuvah 585/7
 Kaf Hachaim ibid as this is no different than confessing between the sets
 Kaf Hachaim ibid