The saying of Yehalelu and Hodu Al Eretz when returning the Torah:
A. The custom:
B. The reason behind this custom:
Some explain that these verses are recited in order to emphasize that we serve Hashem Himself and bow to Him, and not to the G-dliness in the Torah, and therefore we say “Praised be Hashem as His name is alone exalted.” Alternatively, the reason is because before Matan Torah the ray of Hashem was only in Heavens, and after Matan Torah Hashem shines His ray also below on earth.
C. Who is to say it-Chazan, Magbiah, or congregation:
Yehalelu is meant to be said only by the Chazan [of Shacharis] and then be answered by the congregation. [When the Rebbe was Chazan on Shabbos, he was accustomed to take the Sefer Torah and say Yehalelu and bring it to the Aron himself. However, during the week, the Rebbe went back to the Amud and another person recited Yehlalelu and brought the Sefer Torah back to the Aron. In general, the widespread custom today is for the Magbiah or another person to say Yehalelu, and not specifically the Chazan. There is however no reason or source for why some of the congregation are accustomed to also say Yehalelu together with the Chazan.[
D. When to say Yehalelu-before or during the return of the scroll?
Yehalelu and Hodu is recited prior to bringing the Sefer Torah back to the Aron, and only after it is said it is brought to the Aron.
 Siddur Admur “On Monday’s and Thursday’s one enters the Sefer Torah to the Heichal and the Chazan says Yehalelu.”; The saying of Yehalelu is mentioned in: Admur 292:4; M”A 292:2; Siddur Rav Amram Gaon; Machzor Vitri p. 93; Mateh Moshe 256; Rokeiach
 Tehillim 148:13
 Siddur Admur
 Conclusion of Tehillim 148:13 and 148:14
 The reason: The reason why the congregation answers to the Chazan is to emphasize that not only do we praise Hashem, but also the heavens and earth and we then praise Hashem for giving us the Torah and elevating us from other nations. [Mateh Moshe ibid in name of Rav Chizkiya; See there in length for why the verse of Yehalelu is first said by the Chazan and then Hodu by the congregation.]
 Reb Chizkiya brought in Mateh Moshe ibid
 Drashos Maharash, brought in Shaareiy Efraim Pischei Shearim 10:44; Ketzos Hashulchan 91 Footnote 4
 Siddur Admur ibid; Mateh Moshe ibid; Machzor Vitri; Kol Bo, brought in Elya Raba 284:14; Kaf Hachaim 284:44; See Ketzos Hashulchan 25:13 that the Chazan or the person holding the Torah may say Yehalelu.
 Kol Bo, brought in Elya Raba 284:14; Kaf Hachaim 284:44
 Hiskashrus 213 p. 17; 449:18 footnote 43
 See Hiskashrus footnote 10 in name of Rav Y.Y Ofen that when the Rebbe was Chazan during the week the Rebbe let others say the Yehalelu.
 Likkut of Rav Shimon Yakobavitch, printed in Hiskashrus 212:25 who wrote “the Magbia brings the Sefer Torah” although the Rebbe crossed out the word “Hamagbia”
 However, Rabbi Y.L. Groner related to me that the Rebbe was accustomed to say Yehalelu together with the Chazan.
 Machzor Vitri ibid; Implication of Mateh Moshe ibid; Shaareiy Efraim 10:41; However from Siddur Admur ibid it is implied that it is said while it is being brought to the Aron. Vetzaruch Iyun!