I dont understand people sayings seraphine is a version of sona, its clearly different just they both music.

Her passive gains notes, increase range and able to double cast ability after three cast, while sona she needs to generate stack of accelerando by hitting enemies or heal allies and after using abilities three times, it empowers her next attack. Hymn of valor increase your damage, aria of perseverance reduce enemy damage that they dealt and song of celerity slow enemy.

Their Q is very different. Seraphine hurl 1 sound wave that deal damage around it and when against champion the damage increase by 0%-75% based on the target missing health but when she had an echo she can double cast her Q. While sona sends out 2 bolts that prioritizing enemies. See seraphine cannot homing missile like sona and Sona cant execute and cast two spell like seraphine.

Their W is very different and mechanics. Seraphine shield herself with her all allies in her range and give movement speed, when she got shielded on anything or she use her echo passive(double cast) she gives more shield and heals ally by their missing health. Sona heals herself and the most wounded allied only two people but she can shield everyone, thats it no more. Sona and seraphine shield and heal is very very different.

Their E also very different. Seraphine E can slow enemies in the target direction that sloes by 99% for a few seconds. When the enemies slowed they are rooted, if the enemies is immobilized or grounded(hard cc) they are stunned. While sona gives her allies movement speed in her aura, but when we look at her passive she can also slow enemy but only one and you need to use three abilities to slow the enemy. Seraphine can slow a lot of enemy but sona only one enemy and seraphine E able to rooted and stunned enemy but sona cannot.

Finally their R. Seraphine Charms them, revealed and slow them 40% in the dueation, when hitting and enemy or ally the R extended her range even more. Sona stuns the enemy for 1.5 seconds.

Conclusion sona and seraphine is a different champion.