Ni-Mh battery makes fizzy sounds while and after charging - why and do I need t replace it?
It's not possible to give a proper answer to (a) without examining the battery but it is likely due to a compromised casing.  The noise is likely hydrogen gas leaking out as it's produced. Nickel metal hydride (NiMH) batteries use a metal hydride (MH) for energy storage.  When this Hydride is exposed to moisture in air it reacts to form hydrogen gas: $$\ce{MH_x + xH2O \rightarrow M(OH)_x + x H2 }$$

For (b) I would say that your battery is probably not usable—safely or otherwise—anymore and may present a fire hazard. You should replace the battery as rechargeables don't cost near as much as the device they can leak inside of and destroy or the cost of fire damage to a house.