Assalamualaikum warahmatullahi wabarakathuhu.
I have always had troubles regarding periods. My period almost always depends on external factors e.g. yoga/exercise/home remedies. Recently, I came accross a thing called subliminals where u hear some kind of music/tune that changes the frequency of ur brain waves and encourages it to do something i.e. to release particular hormones. It has worked for almost everyone but I wanted to know if it's halal in islam? Considering it's for health purpose only and not for some kinda evil. Also, it has only piano tune in it.

All perfect praise be to Allah, The Lord of the Worlds. I testify that there is none worthy of worship except Allah, and that Muhammad is His slave and Messenger.

It is impermissible to listen to the piano or any other musical instruments under the pretext of seeking treatment and healing. Allah, The Exalted, did not make our healing in what He has made unlawful for us. It has been narrated in a Hadeeth reported by Umm Salamah that the Prophet, said: “Allah did not place your cure in what He deemed unlawful to you.” [Al-Bayhaqi, Ibn Hibbaan: Saheeh (authentic)]

Thus, seeking medication through what is unlawful without a compelling necessity is impermissible.

Another Hadeeth reads: “Allah has created the disease and its treatment, so seek treatment, but use nothing unlawful.” [At-Tabaraani in Al-Kabeer, Al-Albaani: Saheeh (authentic)]

It is mentioned in the Hanbali book Ar-Rawdh Al-Murbi' that: "It (treatment) is impermissible with whatever is unlawful, be it food or otherwise, like the sound of an instrument." [End of Quote]

Shaykh Ibn Uthaymeen said: "Seeking treatment through what is unlawful is not permissible, because the Prophet forbade it. He said: ‘Seek treatment, but use nothing unlawful.’ Moreover, it is impermissible given the general indication of the evidence on the prohibition of the unlawful things; as their indication is general and there is no detailing therein. In addition, if there had been any good in them, Allah would not have deemed them prohibited for His slaves; rather, He would have deemed them lawful to them." [End of Quote]

For more benefit on listening to music for treatment and hypnosis, please refer to Fataawa 120956, 286147, and 323695.

Allah Knows best.