The AudioInjector Stereo card plugs in to the RPi GPIO pins and has high quality stereo sound inputs via RCA jacks. It also has high quality stereo output by another pair of RCA jacks. Prices are in the $20 US range on Amazon and an active forum featuring quick support from the company answers any questions one might have about using the card. I am very pleased with mine and with the support I have received from "flatmax" on the forum. See
http://www.audioinjector.net/. The output quality is equivalent to a good CD.
As for the power issue, if you are using an inadequate power supply, all sorts of problems are possible. The recommended supply for the 3B is 2.5 amps at 5 volts. In most cases, 2.0 amp supplies work well but if you have loaded up your USB ports, you need to get either a good supply or an outboard USB expander with its own power supply. Also consider the connecting cable as it may cause a voltage drop if too long or too small a wire gauge. Power switches in the connecting cable can also cause voltage drops.