That can be a problem... Motors tend to put noise on the power supply and audio amplifiers tend to be sensitive to power supply noise. And, amplifiers that don't use bipolar (positive and negative) power supplies tend to be more noise-sensitive. Digital is SO much easier than (analog) audio!!! 
The best/easiest thing would be separate power supplies. If you were building your own power supply you could use separate regulators and maybe some separate filtering.
Or, here's something you can try -
Add a diode in series with the speaker power (not the motor). A Schottky diode would be best because of the lower voltage drop*, but if you have any diode rated for 1A or more, go-ahead and try it. (If it works you can replace it with a Schottky later.)
Then add a "big" capacitor (1000uF or more) between the speaker-power and ground.
The capacitor will "hold" the voltage and the diode prevents the capacitor from discharging into the motor. That will do a pretty-good job of filtering negative-spikes/noise on the 5V line. But, positive spikes will get through the diode and the capacitor filtering won't be as effective, so it depends on the nature of the noise.
- You'll get about a 0.7V drop across a regular silicon diode and when you lower the power supply voltage you lower the amount of audio power you can get. And, 5V is pretty low for a power amplifier to begin with.