Doctors call the noise your mother is hearing “tinnitus”. It is usually totally harmless, but it can be very bothersome. (Only if it is accompanied by dizziness, consult a specialist to rule out M?ni?re’s disease.)
I’m intrigued by the fact that the tinnitus appeared during pregnancy. Maybe your mother suffered from higher blood pressure at the time, a factor that is known to bring on tinnitus. As we get older, hearing deteriorates, and that can also worsen tinnitus.
Yoga can help because it promotes relaxation, since it is well known that stress increases tinnitus. Look for relaxing yoga classes, like hatha yoga. Or go for guided meditation, like yoga nidra. This is available on CD (e.g. by Richard Miller or SwamiJ.com). Unguided meditation is not recommended in the beginning because it can make one even more aware of the tinnitus sound.
It is possible to mask the sound by running a water fountain or by tuning a radioset to the white noise between stations. In severe cases, an ENT specialist can prescribe masking devices to be worn in the ear.
Another possibility is to do Tinnitus Retraining Therapy (use Google to find the nearest TRT center). This involves education about tinnitus and a process of gradual desensitization. The objective is to no longer react to the sound. Often, the sound is no longer heard at all as a result of the training.
Namast