It is believed that cows like music because it helps them to relax and feel at ease, even when they are potentially stressed. It helps them relieve the stress hormones. Also, the fact that they are generally curious about every other thing around them, music makes them curious and they tend to enjoy it.
Cows can start lactating once they produce a calf. The cow would usually provide milk for ten months after giving birth. And milking can get intensified around 40 to 60 days after calving. After its milking period (in a commercial setup), the cow is given some rest for about 60 days before the calving period.