As always, we preface our answers to such monetary questions by pointing out that we cannot tell you for sure that you are exempt, even if this is our leaning, without formally hearing your friend’s version of the story. However, we will be happy to provide our thoughts based on the narrative that you presented. Since your friend has not demanded payment, you have a right to know if you should honorably volunteer it.
There are two areas of monetary law to consider: One is whether it is possible that you are considered a shomer (guard) and therefore responsible to pay for neglecting to guard the CD properly; the other is whether you are responsible to reimburse your friend under the laws of nezikin (torts) for having damaged her property.
In general, a person is not obligated to guard something and pay for damages stemming from negligence unless she consented, on some level, to accept that responsibility.[1] Since you were unaware that your friend’s CD was in your possession, you certainly did not accept responsibility.
This case, though, is more complicated because this may be a situation where the laws of hashavat aveida[2] apply. It is unclear from your description whether your friend quickly realized she left the CD in your home (and thus it was not lost) or whether she was not sure of its whereabouts and it was an aveida. Let us assume the latter. The Torah imposes the responsibilities of a shomer on one who finds an aveida, even if the finder did not intend to assume such responsibility.[3] The question of whether he is considered a paid or an unpaid shomer[4] is irrelevant here because it is negligent not to put a CD in its case, and all shomrim are obligated to pay for negligence.
Usually, one becomes obligated in the laws of hashavat aveida, including the obligations of a shomer, from the moment he lifts up the object.[5] What happens, though, if one picks up an object without realizing that it is an aveida? There are related discussions on whether one who had physical control of an aveida without fully understanding the scope of his obligation becomes a full-fledged shomer. The Ketzot HaChoshen[6] is unsure whether one who found something he thought was cheap is obligated in its full value when it turned out to be more expensive. This implies that if he had not realized that someone lost the object, he would not have been obligated. Similarly, the Machaneh Ephrayim[7] provides evidence from the Rambam that one who unknowingly has an aveida in his property is not yet considered one who found an aveida. Therefore, in your case, you did not have a shomer’s responsibility.
However, despite not having a shomer’s responsibility, you may be liable for scratching the CD. In general, one who physically damages an object, even with little personal blame, must compensate the owner.[8] Nevertheless, we do not know for sure that you scratched the CD. Something could have fallen on it when it was exposed; your friend could have scratched it; your kids could have done it, and minors – and their parents – are exempt from paying for damages they caused.[9] Therefore, a beit din would presumably not obligate you to pay, since the burden of proof that you damaged the CD lies with the aggrieved party. Poskim discuss whether children who damage should offer compensation when they grow up,[10] and parents often pay for their children’s damages. However, a scratched CD is common; even proper care does not always prevent it. Therefore, you do not seem to have a strong moral obligation to pay, let alone a monetary obligation. On the other hand, you might feel better and make your friend feel good, as well, if you offer her at least partial compensation and see what she says. The choice is yours.