Several pieces of music are mentioned in the novel.

In Chapter 3, Adam declares the perfect logic of playing Bach’s cello suites on the trombone. Helena is listening rather distractedly because she’s contemplating her own progress with the cello suites. She’s already tolerably proficient with the first two suites, but they are progressively more difficult, so it’s the sixth suite that’s occupying her mind:
https://www.youtube.com/watch?v=0SGI1m4YFtA

In Chapter 13, Elizabeth mentions to Miranda that Helena played
some rather mournful pieces of Schubert after breakfast. One of them was Ständchen:
https://www.youtube.com/watch?v=_JpXlliAn2I
Another was Der Leiermann from Winterreise, which would have been exceptionally mournful on solo cello:
https://www.youtube.com/watch?v=QAj06wIdem8
Maybe Elizabeth heard some others ­– please feel free to suggest!

In Chapter 16, Adam remembers Helena being very aminated playing the cello in a ‘string quartet’ – however this is an uncharacteristic lapse of memory on his part – he was obviously so distracted watching Helena perform that he didn’t realise it was actually a string trio that becomes beautifully animated about five minutes in:
https://www.youtube.com/watch?v=nioKJNp8ADE

In Chapter 21, Helena is clearly out of sorts while practising her Kodály sonata. She has acquired her impressive emotional resilience from Elizabeth, but on this occasion she’s weighing the turmoil in her mind caused by Adam. One moment she’s angry with herself for succumbing to her inclinations and then the next she’s reconciled to the peace those inclinations might bring. Time for a walk! – to contemplate temptations – bread, a high place, the power to command …
https://www.youtube.com/watch?v=MyAy6hxGirg