sentirsi and sentire have different meanings.
1) to hear
Lo senti? = Can you hear it?
Stammi a sentire (listen to me!)
2) to feel (with physical feelings)
sento freddo (I am cold)
sento un dolore alla gamba
3) to smell
- sento un odore di gas (I smell gas)
4) followed by a noun it has the same meaning as provare:
- sentire paura/ provare paura (to feel afraid)
5) sentire qualcuno
- to hear from somebody
6) expresses that you have an impression (sense)
- sento che sta per succedere qualcosa
sentirsi is only to feel
I would understand: Non sentiranno bene as They are not going to hear well.