Smelt is an alternative to smelled and is preferable, though not obligatory, in British English. At the end of the day, the sound difference is only between

and

. The verbs that follow a this pattern of having regular / irregular alternatives with this t/d sound ending are:
infinitive irreg past tense and participle regular past tense and participle
| burn | burnt | burnt burned |
| dream | dreamt | dreamt dreamed |
| lean | leant | leant leaned |
| leap | leapt | leapt leaped |
| learn | learnt | learnt learned |
| smell | smelt | smelt smelled |
| spell | spelt | spelt spelled |
| spill | spilt | spilt spilled |
| spoil | spoilt | spoilt spoiled |
In General American English there is a preference to make these verbs regular, which is also acceptable in British English (no one can say you're wrong for using these words) but not usually used. I don't know if, in America, they would tell you that the irregular spelling is wrong or whether it would be accepted as an alternative.