A case in point is the magnificent 1950 recording of the Improvviso ("Un dì all’azzurro spazio") from Giordano's Andrea Chénier, which has never sounded more vibrant. There are numerous other surprises too, particularly among the lesser known (or, in some quarters, less appreciated) late-career gems such as "Just We Two" (from Lanza's 1959 remake of The Student Prince) and the "Drinking Song" from The Vagabond King, both of which now feature a more vital-sounding tenor than heard on previous releases. 
Here is a description of the CD's contents from my liner notes:

"This CD celebrates Mario Lanza’s centenary with a collection spanning the tenor’s entire professional career, from his first radio broadcasts to a quartet of studio gems from the last months of his brief but indelible life. Four of the selections are making their first appearance on a commercial CD; they include a spectacular rehearsal of “Some Day” and extended versions from Lanza’s own archives of material recorded for The Great Caruso. Other rarities include live performances of duets from The Student Prince and Verdi’s Otello. Throughout the CD, Lanza’s celebrated versatility is on full display, with every musical genre in which he excelled represented here." 

UPDATE (12 June 2021): The Immortal Voice of Mario Lanza: A Centennial Celebration is now available directly from Sepia Records for immediate delivery, and will be released in the US on July 16. Here is the Amazon US link for preorders. The Mario Lanza Institute and Museum is also offering this CD (and all of the previous Lanza CDs on the Sepia Records label).

Here is the full track list of the CD:

THE IMMORTAL VOICE OF MARIO LANZA:
A CENTENNIAL CELEBRATION
1 Cavalleria Rusticana: Brindisi  
Recorded August 23, 1950 [longer version of the extract featured in The Great Caruso]                      
2 Ergo Bibamus  
Recorded August 7, 1952  [acetate recorded for the film The Student Prince]                                             
3 Just We Two (from The Student Prince)  [with Norma Giusti, soprano]
Recorded April 1959
4 If I Loved You (from Carousel) 
Recorded February 15, 1952     [with spoken introduction by Lanza]           
5 Long Ago (and Far Away) 
Recorded July 13, 1951                        
6 Some Day (from The Vagabond King) 
Recorded October 28, 1954   [previously unreleased television rehearsal]                      
7 Trees      
Recorded November 20, 1951             
8 The Virgin’s Slumber Song
Recorded May 29, 1950                                
9 Neapolitan Love Song (from Princess Pat) 
Recorded November 29, 1951 
10 La Spagnola 
Recorded May 9, 1952                         
11 Core ’ngrato 
Recorded May 5, 1949                                     
12 Marechiare 
Recorded August 9, 1950 [complete (one-verse) film take for The Great Caruso]                                              
13 Torna a Surriento 
Recorded June 30, 1955                                     
14 Santa Lucia Luntana 
Recorded December 1958                                        
15 L’Alba Separa dalla Luce l’Ombra 
Recorded at June 1959                                    
16 Pagliacci: Vesti la giubba 
Recorded September 1958                                          
17 Andrea Chénier: Improvviso (Un dì all’azzurro spazio) 
Recorded May 18, 1950
18 Tosca: E lucevan le stelle      
Recorded July 22, 1950   [a much longer version of the recording featured in snippet form in The Great Caruso)    19 Otello: Già nella notte densa (Verdi-Boito)  [with Jean Tennyson, soprano]
Live CBS broadcast on November 14, 1945
BONUS TRACKS
20 Golden Days (from The Student Prince)  [with Robert Weede, baritone] 
Live CBS broadcast on February 20, 1946
21 Summertime in Heidelberg 
Recorded April 1959 [revelatory solo version taken from Lanza's private acetate] 
22 Drinking Song (from The Vagabond King)         
Recorded July 1959   
23 Without a Song 
Recorded August 14, 1951​​