For fixed $c,$ the Prolate Spheroidal Wave Functions (PSWFs) $\psi_{n, c}$ form a basis with remarkable properties for the space of band-limited functions with bandwidth $c$. They have been largely studied and used after the seminal work of D. Slepian, H. Landau and H. Pollack. Recently, they have been used for the approximation of functions in the Sobolev space $H^s([-1,1])$. In view of this, we give new estimates on the decay rate of eigenvalues of the Sinc kernel integral operators. This is one of the main issues of this work. A second one is the choice of the parameter $c$ when approximating a function in $H^s([-1,1])$ by its truncated PSWFs series expansion. Such functions may be seen as the restriction to $[-1,1]$ of almost time-limited and band-limited functions, for which PSWFs expansions are still well adapted. Finally, we provide the reader with some numerical examples that illustrate the different results of this work.