Final answer:
You are likely listening to a symphonic poem, a single-movement orchestral work from the Romantic era that conveys a narrative or theme through a programmatic approach.
Step-by-step explanation:
If you're listening to the radio and hear a piece of romantic music for orchestra that is programmatic and contained in a single movement using sonata form, you are probably listening to a symphonic poem or tone poem. The Romantic era saw the emergence of this type of orchestral music, which is designed to evoke a story, theme, or landscape. Renowned composers like Richard Strauss and Franz Liszt contributed significantly to this form, often drawing from literature and other non-musical sources for inspiration. Unlike classical pieces that adhered strictly to form and structure, symphonic poems in the Romantic period allowed for greater expression of emotion and narrative without the constraint of multiple movements typical of symphonies.