Indeed, there is hot debate among Muslim scholars both in past and present times concerning the Islamic ruling of music. The view which is followed by a great number of modern scholars is that all forms of music that are free from sensual themes and messages are permissible. As for the cracking of knuckles, we can say that the mere act itself is permissible but one may do it while harbouring a certain intention as in this case the ruling depends on the intention of the person doing the act. Above all, you should have clarified the reason behind such an act in order for us to verify the issue and furnish you with the suitable answer.

Clarifying the Islamic ruling concerning music and singing, Sheikh Ahmad Kutty, a senior lecturer and an Islamic scholar at the Islamic Institute of Toronto, Ontario, Canada, in which he states the following: “Music is an issue that has been hotly debated by scholars of the past and the present. While many of them have been generally inclined to condemn all forms of music, with the singular exception of ad-duff (tambourine) in weddings, quite a few of them have taken a more positive approach of considering only music containing sensual, pagan, or unethical themes or subliminal messages as being categorically forbidden.
The latter view seems to be more consistent with the general nature of Islam, which is undoubtedly a complete way of life that caters to all of the genuine human instincts and needs within permissible limits. Thus, to say that all music is forbidden in Islam does not seem to agree with the balanced approach of Islam to issues of human life and experience.
Traditions often cited by the first group scholars to justify condemnation of all musical instruments and music, according to some scholars, are considered as either spurious, or phrased in such way solely because of their associations with drinking, dancing, and sensuality.
While everyone agrees that all forms of music that contain pagan, sensual themes, or subliminal messages are clearly forbidden, the latter group of scholars considers all forms of music free of such themes and messages as permissible.
As a matter of fact, we know from the authentic traditions that the Prophet, peace and blessings be upon him, not only allowed music in the weddings but also listened to girls singing: While listening to girls singing on such an occasion, he interrupted them only once when they sang the following verse, “In our midst is a prophet who knows what will happen tomorrow”; whence, the Prophet, peace be upon him, told them, “Cut this sentence out, and continue singing what you have been singing earlier.” There is nothing in the sources to indicate that the above permission is limited to the occasion of wedding, as some people tend to think.
In light of these, according to the last mentioned group of scholars, music that is deemed to be free of un-Islamic and unethical themes and messages, the same is true of musical instruments so long as they are not used for the above, have been considered as permissible.
But we have to stress that Islam clearly prohibits mixed dancing of males and females.”