IANA linguist, but I would definitely agree that emotion is conveyed by a combination of a lot of different things, not the least of which is facial expression (remember all those monkey photos from your Intro to Psych class?). Some other factors (based upon what other people have said above) that come into the mix:
(1) Accent: if we construe this broadly as including volume (such as giving a little extra oomph to syllables that are normally stressed anyway), then sure, it can help express emotion.
(2) Tone: clearly can affect meaning. With different intonations, you can say the word “really” (all by itself) to suggest boredom at or interest in what another person is saying. Some people might not consider boredom or interest to be true emotions (certainly they don’t rank up there with the universals of happiness, sadness, anger, etc.), but they do indicate an inner state of mind. Close enough for me.
(3) Word choice: obviously plays a part, as in “Listen here, bub…” vs. “Listen here, a-hole….”
(4) Elongation of sounds (is there a technical term for this?): as in “I loooove that perfume you’re wearing.”
(5) Non-linguistic utterances: like laughing, or pretending to laugh.
(6) Pace/tempo: Try saying this at your normal conversational speed and see how somber it sounds: “Earth to earth, ashes to ashes, dust to dust.” Not very.
Doghouse Reilly’s example of “bu (rising) yao (falling)” vs. “buuUUU YAOh la!!!”, if I understand it correctly, doesn’t really show a change in tone, but it does seem to involve accent/volume, elongation, and word choice (that “la” ain’t there for nothin’).