Below are recordings of both the Old Spanish and the Modern Spanish of lines 16-39 of Cantar de Mio Cid. Since there are notable differences in pronunciation between the two versions, an explanation of the pronunciation of the Old Spanish version is necessary.
The most notable difference in pronunciation is due to a middle, medieval stage of evolution of palatal clusters; this stage is the affricate stage of evolution, during which time the text was composed.
During this stage, graphemes such as “ce” and “ci,,” as well as “ç” (“c” with cedilla) with “a,” “o,” and “u” and the independent grapheme “z,” yield two possible phonemes. The first is the affricate alveolar voiceless [ts], and the second is the affricate alveolar voiced [dz]. The distinction between these two phonemes is the voicing feature: while [ts] is voiceless (the vocal cords are not vibrated during pronunciation), [dz] is voiced. [ts] arises when the sound is in a protected position in the word while [dz] is formed from a sound in an intervocalic, unprotected position. In Modern Spanish, these two sounds merge into [ts] and then further evolve into the fricative lingua-alveolar voiceless [s]. In some varieties of Spanish, though, the evolution yields the fricative lingua-dental voiceless [θ].
Another difference involves several palatal clusters arising from the graphemes of “j,” “ge” or “gi,” and “i.” These clusters yield two possible sounds, the fricative lingua-palatal voiceless [ʃ] (in a protected position) or the fricative lingua-palatal voiced [ʒ] (in an intervocalic, unprotected position). Again, the only difference between these two phonemes is the voicing feature. In Modern Spanish, both sounds yield the fricative velar voiceless [x].
The last major relevant difference involves the palatal cluster “ll” [LL]. This cluster evolves to the fricative lateral palatal [ʎ] in Old Spanish before delaterizing to become the fricative palatal [j] in Modern Spanish.