Jelly, 138 bytes

ḲḊm2iⱮ“Q®ḃƁ⁼ṇÑ1¿Ọɠ\ƥȥ⁼ṙ⁴EŀƥŒEæʋØ>þ>⁴þʋ[⁾Ƒ]eÞḍ8ß®ⱮȮø⁹ç÷R?g®=,¥ėµ@ØU~"/Xçq®ÆƬ°ɠsṭṭ?dC#Ċ?gѶPÇH5p@ʂEœ⁹½G6ḌƘṪµɦ8€(S¦Ḥ⁸ṫ"ḟḢlṾ¦ƤḃƭẊ@’ḃ4$ÄṬk⁹¤ịØa

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A dyadic link taking the library as the left argument and the Morse code input as the right argument. Returns a Jelly string of the lyrics. The long integer that takes the majority of the code has the lengths of each Morse code character encoded within it.

Explanation

Ḳ                         | Split at spaces
 Ḋ                        | Remove first item
  m2                      | Take odd items (i.e. Morse code representations of letters a..z)
    iⱮ              ¤     | Find index of these in each of the following:
      “Q...@’             | - Base-250-encoded integer 742925387377430937422864458121941352705403867269973650879937105837755604647313120541399315429452997316260711263757137193164236124383957174543961671910365268304120327243209050996915401898167278257845227193061059012165352924148307417330452740236835399667569278751322913192633409885614815
             ḃ4$          | - Bijevtive base 4
                Ä         | - Cumulative sum
                 Ṭ        | - Convert to logical list with 1s at the appropriate indices
                  k⁹      | - Split right argument at those positions
                     ịØa  | Finally, index into lowercase letters

Jelly, 138 135 bytes

ḲiⱮ“Q®ḃƁ⁼ṇÑ1¿Ọɠ\ƥȥ⁼ṙ⁴EŀƥŒEæʋØ>þ>⁴þʋ[⁾Ƒ]eÞḍ8ß®ⱮȮø⁹ç÷R?g®=,¥ėµ@ØU~"/Xçq®ÆƬ°ɠsṭṭ?dC#Ċ?gѶPÇH5p@ʂEœ⁹½G6ḌƘṪµɦ8€(S¦Ḥ⁸ṫ"ḟḢlṾ¦ƤḃƭẊ@’ḃ4$ÄṬk⁹¤ịØa

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A dyadic link taking the library as the left argument and the Morse code input as the right argument. Returns a Jelly string of the lyrics. The long integer that takes the majority of the code has the lengths of each Morse code character encoded within it.

Explanation

Ḳ                      | Split at spaces
 iⱮ              ¤     | Find index of these in each of the following:
   “Q...@’             | - Base-250-encoded integer 742925387377430937422864458121941352705403867269973650879937105837755604647313120541399315429452997316260711263757137193164236124383957174543961671910365268304120327243209050996915401898167278257845227193061059012165352924148307417330452740236835399667569278751322913192633409885614815
          ḃ4$          | - Bijevtive base 4
             Ä         | - Cumulative sum
              Ṭ        | - Convert to logical list with 1s at the appropriate indices
               k⁹      | - Split right argument at those positions
                  ịØa  | Finally, index into lowercase letters

Jelly, 138 bytes

ḲḊm2iⱮ“Q®ḃƁ⁼ṇÑ1¿Ọɠ\ƥȥ⁼ṙ⁴EŀƥŒEæʋØ>þ>⁴þʋ[⁾Ƒ]eÞḍ8ß®ⱮȮø⁹ç÷R?g®=,¥ėµ@ØU~"/Xçq®ÆƬ°ɠsṭṭ?dC#Ċ?gѶPÇH5p@ʂEœ⁹½G6ḌƘṪµɦ8€(S¦Ḥ⁸ṫ"ḟḢlṾ¦ƤḃƭẊ@’ḃ4$ÄṬk⁹¤ịØa

Try it online!

A dyadic link taking the library as the left argument and the Morse code input as the right argument. Returns a Jelly string of the lyrics. The long integer that takes the majority of the code has the lengths of each Morse code character encoded within it.

Jelly, 138 bytes

ḲḊm2iⱮ“Q®ḃƁ⁼ṇÑ1¿Ọɠ\ƥȥ⁼ṙ⁴EŀƥŒEæʋØ>þ>⁴þʋ[⁾Ƒ]eÞḍ8ß®ⱮȮø⁹ç÷R?g®=,¥ėµ@ØU~"/Xçq®ÆƬ°ɠsṭṭ?dC#Ċ?gѶPÇH5p@ʂEœ⁹½G6ḌƘṪµɦ8€(S¦Ḥ⁸ṫ"ḟḢlṾ¦ƤḃƭẊ@’ḃ4$ÄṬk⁹¤ịØa

Try it online!

A dyadic link taking the library as the left argument and the Morse code input as the right argument. Returns a Jelly string of the lyrics. The long integer that takes the majority of the code has the lengths of each Morse code character encoded within it.

Explanation

Ḳ                         | Split at spaces
 Ḋ                        | Remove first item
  m2                      | Take odd items (i.e. Morse code representations of letters a..z)
    iⱮ              ¤     | Find index of these in each of the following:
      “Q...@’             | - Base-250-encoded integer 742925387377430937422864458121941352705403867269973650879937105837755604647313120541399315429452997316260711263757137193164236124383957174543961671910365268304120327243209050996915401898167278257845227193061059012165352924148307417330452740236835399667569278751322913192633409885614815
             ḃ4$          | - Bijevtive base 4
                Ä         | - Cumulative sum
                 Ṭ        | - Convert to logical list with 1s at the appropriate indices
                  k⁹      | - Split right argument at those positions
                     ịØa  | Finally, index into lowercase letters